So You Want a Crystal Clear Pond

Published on June 1, 2023


This post was originally published in 2014.

Discover a (nearly) maintenance-free system for a crystal clear pond

When I was asked to write about what it takes to have a crystal clear pond, a lot of ideas came to mind. This is a topic I could easily write an entire book on and
still only scratch the surface. So instead of glossing over all the factors contributing to a crystal clear pond, I will focus on the most important one: filtration.

There are several different types of filtration, but the two that will have a major impact on the clarity of your water are mechanical and biological. While both of these filtration types can be man-made or naturally made, I am going to cover man-made filtration specifically.

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Of course, just because we are building the filter doesn’t mean that we can’t use natural materials for the media. Thus, I’ve chosen to write in detail about a natural media that, in a lot of circles, may be considered a dirty word: rock and gravel.

Rock and Gravel

Almost every gravel area that you see is either an undergravel grid filter or upflow gravel filter. Everything is run off air lifts.

A very old type of filtration media, rock and gravel were used for a long time but have lost favor with many ponders. One of the reasons for the falloff is that the surface area per cubic foot is not very high — or so people think. Yes, rock is solid material that takes up a lot of space, but it is also a natural material that’s formed on a very fine structure and then
is eroded on a microscopic level. With that in mind, the surface would have a fairly high microscopic surface area,
which is never included when talking about gravel surface area.

Even taking that into consideration, a filter using rock or gravel will require a larger footprint to handle the same size pond as some of today’s newer medias. But the smaller the filter, the more frequently it requires maintenance. Thus, the small amount of maintenance required to maintain many rock or stone filters is not easily accomplished with other media.

Rocks on the Bottom

When we talk about putting rock and stone on the bottom of the pond, we immediately stir up passionate feelings in some people. There is a lot of debate about
whether a pond should have stone on the bottom, and the argument boils down to the buildup of debris in the gravel.

To speak to this issue, let me tell you about a pond that I am very familiar with — a pond I built 22 years ago! This pond is six feet deep and has about six inches
of gravel on the bottom that has never been cleaned.

Now, I know what you’re thinking: “There must be a whole lot of debris in that gravel.” But when I swam in the pond last year, I dove to the bottom and dug in the gravel … and there was no debris to be found! How is that accomplished? The secret is an undergravel suction grid system. If designed correctly, it performs
excellently with very little maintenance.

Undergravel Suction on a Small Scale

An undergravel grid is being installed in an existing pond. The old gravel is being reused.

What is an undergravel suction grid filter? Well, most of you are familiar with its smaller counterpart: the undergravel suction filter in aquariums. A filter like this is built with thin slots in the plate, which is supported off the bottom of the aquarium. Small gravel is placed on top of this plate, and water is sucked through
the gravel and the slots.These filters work great for a while, but then they need to have a lot of maintenance done or they fail. By “maintenance” I mean the gravel on top of the filter suction plate has to be cleaned.

Anyone who has done this maintenance on his fish tank knows it is not a funchore. In a busy aquarium full of life, waste and debris quickly build up in the slots, causing the gravel to clog up faster than you can (or want to) clean it.

Ponds vs. Aquariums

If we want this type of filter to work in a pond, we have to look at why it has problems in an aquarium.

The real difference between the undergravel grid in a pond versus in an aquarium is the space in the grid. In the aquarium, the space between the pieces of gravel and in the slots in the suction plate is very small compared to the waste produced. In order for the system to allow water to flow through and not clog up, the waste has to be almost completely eaten by bacteria. This process takes longer than it takes for the gravel to clog up. Therefore, the filtration simply cannot keep up with the waste. The filter clogs up and fails.

An undergravel grid installed and ready for gravel.

To avoid this problem in the pond I built, I designed the undergravel filter to have a series of pipes on the bottom with ⅜-inch diameter holes drilled in them.
The pipes were buried in .- to 1-inch round gravel with about two inches above the pipe. The spacing between the pieces of gravel is fairly large, and the holes in
the piping are large compared to the waste to be broken down. All the holes in the suction pipes are six inches apart, providing the waste a lot of area to fill.

In this grid there are six inches in every direction that would have to clog up before this filter would need to be cleaned on every suction line. Of course, if this filter were only a small part of the bottom of the pond, then there could or would be enough waste to clog the grid before the bacteria could break it down enough to get rid of it. Therefore, the larger the area of the pond’s bottom that can be part of this filter, the better.

Will this filter ever clog and need cleaning? The answer is yes, but the better question is: How long will it take? There is no set answer. It all depends on how much debris or waste is being put into the pond (or being made by the pond).

Success Story

Earlier I wrote a little about the 22-year-old pond with a gravel bed that has never been cleaned. This happens to be my own koi pond and my design. Located in Batavia, Ill. (about 35 miles west of Chicago), this pond was built to be as maintenance-free as I could make it.

No, it is not completely maintenance-free … but it is close. It is about 18,000 gallons and requires an average of three minutes of maintenance a week. None of
the maintenance is spent on the undergravel suction grid filter. The pond has never been emptied or cleaned since it was built. Based on what I have seen, the filter
will not clog up as long as I am alive or as long as the liner lasts. My guess is that the liner will last for another 25 years.

I did make a mistake when I designed and built this pond. I used .- to 1-inch round limestone gravel. The problem is that after 22 years, the limestone gravel
is shrinking in size. I believe I may have to remove this gravel and replace it with gravel that takes longer to erode.

Of course, I never thought that the filter would go this long with no maintenance required. Sometimes you stumbl on the right combination of ideas and designs and things work far better than expected! Is it working in Illinois because of the climate but possibly would not work elsewhere? I would say no; I just returned
from California, where I saw a pond that’s about six years old and has one of these filters. It is over-stocked with koi, and they eat well. But the grid is working great and has not been cleaned. I saw this pond two and a half years ago and it looked good then — but it looks even better now.

Half the bottom of this pond is undergravel suction grid, and the other half is undergravel pressure grid filter — all being run off submersible pump.
Half the bottom of this pond is undergravel suction grid, and the other half is undergravel pressure grid filter — all being run off submersible pump.

Will this design work in every situation? That I can’t answer, because this type of filter hasn’t been used in every
possible circumstance there is. But it has worked perfectly every time that I know of it being tried!

I have also used this system as a pressure undergravel grid filter, and it has performed perfectly for the last seven years. The only complaint from that customer is that the water is too clear. A suction undergravel grid filter normally uses an external pump or air lift system to run it, but a suction filter can also use a submersible pump. A pressure undergravel grid is built similarly to the suction grid, but water is pumped through the grid. The pond that is pictured above uses both suction and pressure undergravel grid filtration. The picture was taken when the pond was five years old. Both systems are being run off the same submersible pump. This is a very formal pond and we kept everything inside the pond.

Versatile and (Almost) Maintenance-Free

In conclusion, the undergravel suction type of filter I’ve described will give you a great mechanical and biological filter. It is hidden in the pond and doesn’t require an area larger than the pond. If done correctly, it has proved to require little, if any, maintenance.

It can be used with either external or submersible pumps. It also works great with the latest air lift technology. I don’t know if “the perfect filtration system” really exists. But this one comes close enough for me.

Mike White is the owner/operator of White Water Filters LLC and Sue Miller Enterprise in Batavia, Ill. He built his first pond in 1990 and instantly fell in love with the hobby. In 1995 Mike became president of the Midwest Pond and Koi Society, the second largest pond club in the country. In 1998 he started White Water Filters, a pond construction company.

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188 thoughts on “So You Want a Crystal Clear Pond”

    1. We took a fire to rout in 2005with a waterscape and rock feature with an upper, ‘bog’ pond dug 8’ with liner that the water from the pond below is pumped to the bottom of with a access to drop a sump to the bottom to pump out the bog if necessary. Starting with hundred pound boulders in the bottom the stone on stone diameters reduce/stone surface area increases until it tops out with a mix of river rock/gravel up through which the water enters a stream that splits into right and left arms falling to the pond the left falling over a cantilevered flat stone enhancing the sounds of falling waters on one side and more gently through a split rock face onto a shallow rock pad with ornamental cranes and cattails on either side. The streams highly oxygenate the water before it drops into the pond which has a river rock bottom. With the streams running 8 hours a day the water remains clear and the pond clean enough to swim in. The koi appreciate it on hot days.

  1. I have a few questions about the picture of the very long and narrow “under gravel grid installed and ready for gravel”. What size pipe did you use? Did you also drill three 3/8 inch holes every 6 inches as you described in the 22 year old pond that you built? What size pump did you use? It seems to me that it would take a huge pump to create enough suction for all those holes and for that large of an area. Can you expound on that?

    1. Hi Suzanne, I did not build that long narrow pond that is pictured. That was Eric Triplett “The Pond Digger” out of California pond. I did help with the design of it. Yes it was made with 3/8″ every foot. The three inch pipe was used to handle the flow from the pump. On Thursday of last week Eric was out here to take a underwater video of that now 24 year old pond. The problem was that Wednesday night we had a bad rain storm and the pond pumps lost power all night. Between the 2 inches of rain we got and pumps being off when everything got started back up at 7 am the pond was pretty cloudy when Eric got here at 7:30 am. He said he would have to come back and that the video. I took him out to breakfast. When we came back less then a hour later the pond was clear and he took the video.

      1. So you have the bride connected to your pump to pump out the waste?
        Could a person make a grid and not put gravel on it. Just leave it in the bottom of the pond?

    2. Jeremy Shane Schaefer

      I’m interested in building one in my back yard. I would like to have more information on this or is there anyone that puts these in and a ought how much I’m in the country, Brazoria, Texas. I have the equipment to dig it just need to know what all to use. I have 5 kids so not a lot of extra cash and I can do most of it myself. I just need to know where to start. I like the sound of the filter system and rock bottom, just looking for something to float around in. We swim in the pond but the perch keep nibbling, lol. The girls don’t like to hang out in the water long, so only jumping in and getting out. Can’t really relax, so I’m looking to build one closer to home that we can enjoy. Any and all suggestions are appreciated. Thanks.

      1. I don’t believe in building a swim pond. If you want something to swim in then a swimming pool. There is too much liability involved with a swim pond.


  2. Avatar photo
    Lora Lee Gelles

    From Mike White:
    The size of pipe is 3″ PVC with 2″ lines going off to the sides of the pond. The holes are 3/8″. The surface area of all the holes drilled should be at least 5 to 10 times the surface area of the cross section surface area of the pipe. Each 3″ line has it only pump running the line. Each pump is moving about 7000 gph. The size of the main suction lines is determined by the amount of water the pump is moving and the volume of the pond. The nice thing about this system is that it is a self adjusting system. The flow rates at different points in the system change constantly. Where the flow rates are higher more debris will be sucked causing more restriction of the water movement causing the water flow to change somewhere else.

  3. I have a couple questions.
    Are there holes drilled in the central suction pipes, like the cross tubes?
    Do the holes in the pipes face up or down?
    I have a pond that is about 2600 gallons, built about 12 years ago, and want to upgrade all the filtration systems, and your help would be greatly appreciated.
    Hindsight is 20 20, wish I new this info back then!

    Ed OBrien
    New Jersey

    1. Avatar photo

      From Mike:
      Yes there are holes drilled in the central suction pipe. All holes are drilled in the bottom of the pipe. The reason for this is so the system can remove debris all the way to the bottom of the pond. Also it uses the entire bed of gravel. This way the gravel will stay clean as there is no where that doesn’t have the potential of water moving through it.

      I like to have as many holes as possible. I like to have the area of all the drilled holes being ten times the area of the cross section of the main suction line. This allows the filter to automatically adjust it self depending on how much debris is in the pond. The one thing that is difficult to control and is hard on the system is leaf debris settling to the bottom. This can effectively block the flow of water through this section of the filter. In the northern climates this usually happens when the system is either shut down or going to be shutdown soon.

      1. Hi Mike,
        What is the space between each hole drilled in the main pipe? Is it still using the 3 hole principle which is at 4, 6 and 8 o’clock position for the main pipes? Thanks.

  4. I’d like to know if I have a 4000 gal pond, with an 8500 gph external high head pump, would I have to have a separate pump for the under gravel filtration system from my waterfall? The pond is 50ft long by 3.5 ft wide and 2-3 ft deep along most of it. Top of falls is 5ft above waterline and need about 5000gph coming out of a 32″ diffuser. Appreciate your input!

    1. Hi Karl, I am not sure if I understand the question. The under gravel grid filter would go to the intake side of the pump. Would you require a second pump? That would depend on what else would you want this pump to run. I would want the 5000 gph to run the suction grid at a surface area of about 175 sq ft. Having an 8500 gph “high” head pump I would not want to drop the head pressure down to get the 8500 gph that the pump might be able to pump as the pump motor will over heat.


  5. I’d like to know what you are doing to catch all the debris being pulled out? Basically, what are using, settling chamber or some type of material removal?
    Jeff Higgins

    1. Hi Jeff, We use skimmers to remove surface debris. Sticks or branches have to be physically removed. Most other organic debris will break down by itself and not be a problem.


  6. Great article ,Mike. I have a customer with s very formal pond , similar to the one pictured. I’m going to show him your article and try to sell him on your system.

  7. Hi Mike,
    Thank you! I currently run a gravity return bog/natural pond that has been a huge success but am moving to a self contained above ground tank and want to use the undergravel grid filter, all contained in tank if possible. This is terrific info on the grid construction, thank you! I just can’t seem to get clear on in and out points of water & suction vs pressure (using submersible pump). Can you help?

    1. Hi Tymber, Yes it can be hooked up to a submersible pump. You could connect the suction grid piping directly to the intake of a submersible pump. But that is asking for problems. You will probably get something bigger then the pump can handle entering the pump. You might be able to pipe a leaf basket used on external pumps to catch anything before the pump. What I usually do is use a pump vault that I can seal. I put the pump in that and then pipe the grid to dump into the vault. The output of the pump is then piped thru the vault to where I want the output to go. When the pump is turned on then the only water the pump can pump is water that is getting into the vault from the grid. The only real problem with using a submersible pump is when they have to be changed. It can be difficult to remove and replace the pump underwater. I have one pond that I installed 10 years ago using this system that is functioning perfectly.


  8. I have a pound about 10 ‘x15’ 2 ft deep I understand the pipe lay out confused on type of pump to use and where is the the debri pumped to can you show pics of the pump part of this system

    1. Hi Wayne, Normally an external pump is used with this system in which the plumbing is simple. The underwater grid piping goes out of the pond and then connects to the intake of the external pump. It can be connected to submersible pump but it is a little more difficult. See my response to Tymber question.

  9. Hi,

    First thanks for a great article 🙂

    Have been thinking about updating my pond to something like this and now that I have a leak, it seems like the perfect time…

    Have always had 100% clean water = I can see small stones on the bottom of the pond almost 2m down..

    My pond is split up in 2, I have a root zone (lava rocks and plants) size 180x220cm and it’s 40cm deep, here all my water pass after the filters and then ends up in the main pond size 190x220cm and is 190cm deep. What I like to do is to use less mechanical filters = only keep my Laguna Pressure Flo 4000 as it is very easy to keep 🙂

    Questions that I hope you can answer:

    1. What amount of gravel do I need to add on top of the pipes, if I add this to the main pond?
    2. Will my pond be a good fit for this type of solution if I ask you?


    1. Hi Jacob, I will answer the second question first. The pond being 6.25 ft x 7.25 ft the underwater grid filter will work perfectly as long as the piping is sized for the water flowing through it. The amount of gravel is determined by the size of the pipe used in the grid. In your case you might use 1.5 inch pipe for the grid. The outside diameter of that pipe is a little bit smaller then 2 inches. Assuming the pipe lays on the bottom then you would need a layer of gravel 4 inches thick, O.D. of pipe plus 2 inches on top of pipe. In your case about 15 cubic feet of gravel.

  10. We just have highly alkaline lime stone gravel in my state and any other like lava rock or river rock cost 15 to 30 times more. Its fine to use lime stone? thanks for your help.

    1. Hi Marco, Limestone gravel is not my first choice. The pond water will slowly dissolve the limestone gravel. If the water being used in the pond is not normally hard the limestone will make it that way. My own pond I used Limestone gravel 3/4 to 1″ round. The hardness of the water used for the pond is 450 ppm so the gravel didn’t change it.

  11. Hi Mike, I just have two questions.
    What kind of gravel do you suggest using?
    What would be the optimal gravel size?

    1. Hi Cameron, My original pond that I built 25 years ago I used 3/4 to 1″ limestone round gravel. It worked beautifully except the water flowing thru it was dissolving the gravel. Last year I had to put a layer of 3/4 to 1″ round granite on top of it. I normally use 3/4 to 1″ round granite gravel. I did build one formal pond with pea gravel. That was 10 years ago and it is still working perfectly. The hole sizes and number of holes had to be modified.


      1. Mike, planning on building a 9x7x4 koi pond. Would like you to give me piping manifold for gravel filter. Have 4500gph pump. Thanks, Brett

    1. Hi Ben, I have never used fractured gravel before so I do not know. I would be afraid that there might be problems because of the small spacing between rocks. The one time the customer wanted me to use pea gravel I was afraid that it would fail but that was 10 years ago and it is still working with no problems. So I do not know how it would function. If you try it let me know.

  12. Thank you Mike. I have been trying to find ways to make my fishpond water clear. Barley straw, the chemicals, and submerged small filter boxes have not worked . I want to try an external pump. What kind of pump are they?, what do you suggest?
    Finally: do I still need to use some sort of sponge type filter in addition to the gravel?. Thank you again

    1. Hi Hamid, As far as a pump goes it depends on how much water you need to pump at what head pressure. I am not going to suggest a brand pump. There are a lot of different brands out there and some work better as far as how they will perform on your circumstances.
      On any external pump you are going to want a leaf basket before the pump to protect the pump. Some pumps have the built in. If the pump is above the water in the pond then you are going to want either a self priming pump or a good check valve in the system so the pump will not loose its prime and damage the pump. I am sure there are articles that will help you decide on what pump to get for your circumstances.

  13. Mike, first off, thank you! This is a game changer.
    Do you have a basic rule of thumb/ratio to determine gravel depth, pipe size, holes and pump size based off the total pond volume? Secondly, would this need to be ramped up if the pond were to contain koi? Not a crazy amount… say 20 gallons for every 1″ of fish as opposed to the basic 10.

  14. Hi Rando, I usually set the pump size to handle the volume of the pond once an hour. If this is not the only filter then the pump size can be smaller so that the volume of the pond goes through all filters in an hour. The gravel depth is determined by the pipe sizing in the pond. The pipe sizing is determined by volume being pumped or moving thru the pipes by an air lift. The depth of the gravel is 2 inches over the top of the pipe on the bottom. This can easily handle koi.

    1. Thanks Mike!
      Could I use larger, like 2-3″diameter, flat river stones? Or perhaps a combination things like lava rock on the bottom most layer, 2″ of 3/4-1″ gravel on top of the piping system, then like and extra inch or two of 2-3″ diameter on top of the gravel? –looking at 3 things with this direction.
      1. Additional filtration
      2. Aesthetics (flat river stones)
      3. Preventing koi from digging in with the size and weight of the flat river stones.
      I’m specifically interested in creating an undergravel pressure system with “suction” chamber in one end, taking in water from mid to surface depth and pumping water back in through the gravel from the bottom up. Id love to hear your thoughts on this idea.

      1. Hi Rando, I gather you are wanting to create a pressure bottom system. Yes you could do what you are wanting to do but it is not going to filter as well. In a pond there are three kinds of debris that you want to remove. One is lighter than water and is removed at the surface. Another is neutral buoyancy that is a lot harder to remove because it can be anywhere in the water column. And the last is heavier than water and will sink to the bottom. An under gravel filter is designed to remove the latter. What you are wanting to do is to use it to remove lighter than water waste. I have never done this so I do not know how it would work. But I can see all kinds of problems.

  15. Thanks Mike! Just a few follow up questions…
    Would 2-3″ flat river stones work? How about a combination of things like porous lava rock for a bottom layer, 1″ gravel ’til 2″ over the piping system and lastly, another inch or two of the flat river stones? I’m curious about the flat river stones for both aesthetic and functional purposes, specifically, having a protective layer over the 1″ gravel to prevent possibility of the koi digging into it. The set up I have in mind is a pressure under gravel system with the pump in a chamber in one end and water flowing into it from mid depth. Please let me know your thoughts on this as well.

  16. Hi Dave, I would guess the volume of that pond would be 8 to 9 thousand gallons. I would like to pump 5 to 10 thousand gallons per hour. I know that is quite a range but without knowing what the footprint of the under gravel grid will be I can’t be more exact. But lets say it is large enough to move about 8 to 9 thousand gallons per hour. Your main feed line should be a 3″ line with 2″ lines as branches. The pump should be large enough to move that amount of water to the head height that the water is being used for. You might even use two smaller pumps that combined can move that water. This might reduce electrical cost.

  17. Hi Mike, I have started my pool/pond and it is a little bigger than i first was thinking. I’m putting everything together out in the yard before I do any digging so once the liner is in I’ll be ready to go. The pond will have 4 4″ airlifts around the outside of the pool area walls but want to gravel the bottom of the pool so would like help designing the under gravel suction grid. I could email you a rough sketch of my project if you would be willing to help. Thanks and have a great day, Dave

  18. Mike, this sounds great.
    I am upgrading my tiny pond to a 20’x10’x4’. It should be a little shy of 6,000 gallons.

    Do you prime and glue the pvc together?

    I will run 3” pvc pipe down the middle the long way.
    Then run 2” pvc all the way to each side spaced every 12” Right?
    Holes are drilled on the bottom of pipes every 6”

    -external, 6400 gph pump and a skimmer

    I have not been able to find gravel I like under 1”
    The best I have seen is 1”- 1.5” round river type rock.

    I really appreciate the article.

    1. Hi Jim

      20 x 10 x 4 pond is a decent size. But I would guess you will end up with about 4000 gallons of water. But that is still a good size. You have everything correct except the holes in the pipe. They should be about 12 inches apart with a hole on the bottom and a hole 60 degrees up from the bottom on each side. So you end up with 3 holes every 12 inches. The rock is a little large but will work but I might want to have the rock deep enough to be 3 to 4 inches above the pipe.
      Have fun.

      1. Mike don’t you think system would work better using pressure pipes with quick release couplings to clean out pipes when required and using a grinder to cut slots into pipe rather then drilling as well as using a media called lytag to cover the pipe work?

        1. Hi Glen,

          I have never seen a time when any of the piping needs to be taken apart. I am not sure what you mean by pressure pipes. Schedule 40 PVC piping is perfectly fine. What you are trying to do is make this system like an under gravel system for an aquarium. If you do it will fail like an under gravel aquarium system does.


  19. Really glad I found this article. We have a sizable pond that we want to revamp the filtration system. The pond has a heavy bio load due to the duck population. We have attempted a bog filter with limited success because the ducks destroy the vegetation. This underground setup appears to be solution I’ve been searching for! I don’t quite understand how the under-gravel pressure grid filter works with this under-gravel suction filter. Can you please explain? Thank you!

  20. With the under gravel pressurized system the piping is almost like the under gravel suction system. The only difference is that there are no holes in the bottom of the pipe but only off to the sides of the pipe. The way I do it is to use half of the filter for suction and half for pressure. The suction pipe is run to a container that can hold the submersible pump. The pipe from the suction side is run through the wall of the pump container so that when the pump is running the only place it can get water is from the pipe going through the side of the container. In other words the container must be sealed so the place the pump can get water is from the suction pipe. The output from the pump would be connected to the piping for the pressurized under gravel part of the filter.

  21. Hi Mike, I have some questions, but first a brief on my diy project. Main pond is 20,000 gallons. For it I have built a filtration bog pond (surface area ratio 1:1) out of waterproof concrete, additionally waterproofed with koi safe rubberized bitumen. There is a 4 inch outlet pipe from this bog pond to be connected to the under gravel filter for gravity feed into a sump, no suction or pressure. The sump is to house a submersible pump. The gravel is to be around 1 foot deep, water depth sloping from zero to 18 inches. maybe up to 1’6″ with top to bottom downward flow to the piping. Gravel to be planted with various aquatic plants for root and bio filtering in the gravel.
    Now the questions part is about the actual under gravel filter piping.
    What diameter should my gravel filter piping be? What size and spacing of holes or notches? Should these holes face up or down, as some claim facing down clogs the holes? what gravel, pea gravel (0.5 inch) or three quarter inch? Is a variable speed motor/pump a good option? Is there a pump/flow calcultation formula? My apologies for the long winded post. Thank you.

  22. Hi Cliff, I don’t know if I understand the setup. I believe the sump that you talk about will be in the bog. Then the question has to be how is the water that is taken out of the bog replaced. How big a pump are you going to have in the sump and what actual output will this pump have. How far can the water in the sump drop before there is a problem. How far is the water going to have to travel to get to the pump in the sump. As an example if the 4″ pipe to the sump is 10 and the water level is 2 inches lower than the bog then 9500 gph is being pumped provided that the water going into the bog can keep up. I would want 3/4 to 1 inch gravel and plants with short root systems so they don’t clog up the gravel. I will need more information to give you much more information. As far as to is there a pump/flow calculation formula. I have put together charts that give water flow thru pipe based on the difference in the water level for one body of water compared to the other body of water connected by PVC pipe.

  23. Hi mike…i am making a fish pond of 2000 gallons with 3 feet depth..i am using 1.5 inch main pipe and side branches of 1 inch pipe.i will make three filters in three corners of the pond and three submersible pumps of 10000 litre each would be fixed on three uplift pipe of 1.5 inch reduced to 1 inch…where am I wrong please quid’s me.regards.nizami

  24. Hi Zia, If I am understanding this correctly you are going to connect three 2500 gph submersible pumps to the grid. I don’t know if you are planning three grids or the same grid being sucked from three different points. The other thing I don’t know is where is the water from these three pumps going to go to have some idea of what head pressure they will have to have a sense as to how much water there are going to move. If you are moving the full 2500 gph then I would want to up size the piping to 2″ to 1.5 inch fingers. I would probably do this in any case as pipe is cheap and it is easier to drill the holes in large pipe. But in any case the system should work wonderfully with that amount of water moving on that size pond.

  25. Robert Mountain

    Hi Mike… Thank you for posting this. I have been designing a turtle pond for my wife. These are messy animals. The area we are working with is small. The pond is 6 feet by 3 ½ feet by 2 ½ feet deep with straight walls and a flat bottom. There will be blocks raised for plants and basking platforms. There will also be a ramp to allow access out of the pond. I am designing a 1 foot by 4 foot bog filter on top of a waterfall. After seeing this article I am adding the under gravel filter to the pond design.
    The pond is about 400 gallons. I have a 900 GPH submersible solar pump for the waterfall. I also have a 400 GPH submersible inline solar pump to connect to the gravel filter. My grid will be 1 ½” main with 1” fingers spaced 1’ apart. The grid is 22 total inches of pipe with holes 6” apart and 4 inches of ½” to 1 ½ river rock. The 400 GPH pump will be connected to a Venturi jet about 3” above the rock bottom to add air and circulation to the bottom of the pond. I know the bog filter may be overkill but my wife already bought the plants. Does this system look okay? Should the fingers be 6” apart?

  26. Hi Robert… I would still drill .375 diameter holes but I would space them every 6″ apart and install fingers every 6″. If I could find gravel that is more uniform as far as size that would be better. The problem with the size you have is it will pack together tighter. The system should work.

    1. Hi Mike, I wanted to leave an update. We completed the pond in July 2018. By September the pump had gone to a trickle. I removed the 1 1/2 in grid and replaced it with 2″ pipe. instead of drilling holes I used my circular saw to cut slits in the underside of the pipes about 2 inches apart to allow good flow. I built a 2 square foot bog next to the pond 1 foot deep with the same 2 in pipe grid as the pond and a waterfall going into the pond. This has been running since September without issues. I have a clean pond good water circulation. For those looking to build a pond with the underground filter, even though it is a small pond, use big pipe! Thank you for your help Mike! Great article.

      1. Lawrence Hughes

        I am building a pond that is 25’x15′. I was planning a building a multi-grid using a 3″ pvc with 2″.fingers every 12 inches. Holes every 6″ . I was planning to tie the grid together but still have 2 3 inch pipes using air lift to take the water to the settlement chamber. Was also gonna put 1-2 inch to we rock on bottom and 1 in river rock above pvc for about 3 inches. Does this sound like it would work

        1. Hi Lawence,

          Holes should be 1 ft. apart so that each set of holes can suck 6 inches in all directions that there is rock to move water thru. If the air lift system is built correctly then it can and will move enough water thru this filter. I am not sure why you would have this water go to a settlement chamber as this chamber will stay clean without having to do much.


  27. Hi Mike,
    do you have any experience with a reverse filtration, where the water is taken from the surface into a sand filter to remove larger particles and then pumped through a perforated pipe which is wrapped with a needle felt fabric. The pipe is then covered with gravel (about 3 inch thick) and then with river stone (about 2 ich thick). Instead of pumping the water directly into the pipe system I pump the water into a 2000lit tank using a venturi to mix the water with oxygen. From the tank the water is flowing by gravity into the underground pipes and from there seeps slowly through the gravel and stone bed into the pond.
    Please give me your opinion.

  28. Hi Helmut,
    I am not sure I understand your system. I am not a fan of using a sand filter on a pond. The bio load in pond is too much for the sand filter as the pond gets older. Then I would guess that the fabric on the pipe will clog up and then with no pump moving water through it. But if it doesn’t clog up it should work.

  29. Mike,
    Can you post a representative photo of the gravel size you recommend? It seems very important to get the right size…..I am just trying to make certain I am visualizing this correctly.


  30. Hello,

    Sounds like an excellent system. The only thing not clear to me is where does the water that is suctioned out through the grid go before returning to the pond; Mechanical filter? Bio filter? Bog?
    Please clarify this.

  31. Dietrich,
    That water can go any where you want it to go. The water will be clear of any debris, the ammonia and nitrite will be removed. So you could send it to another filter if you want but not necessary. If you wanted to send it to a plant bog to remove the nitrate you could. If you want to use the water for a waterfall or stream you could.

  32. Hi,
    I’m considering converting an old inground concrete swimming pool with a very deep end. I’d like to add rock to the bottom to make it a bit shallower from 10 t0 about 4-5 feet or so. Can I use the rock bottom and under gravel filtration system without adding a liner? I’m concerned about a deep rock bottom becoming a swamp. Any info appreciated

  33. Hi Deb,
    This is one of the problems with converting a swimming pool. If you just put rock in the pool to make it shallower you will 5 to 6 ft of rock in the bottom that will fill up with debris that can’t be cleaned out. Why not keep the depth and put the under gravel filter on the bottom of the pool. The fish will like the depth.

    1. Thanks for your reply. Is there something like a an open cage that place over the filtration system but would decrease the depth of the pool but not hinder filtration? I’m thinking about safety for kids or wildlife. If really like to avoid tearing out the pool and starting over.

  34. Hi Mike

    I am building a 12×20 54 in deep pond using the helix skimmer and waterfall. I put the liner in and noticed after it rained that the bottom was out of level by approx 1 in to 1.5 in over 20 ft. I am doing the grid system out of 2 in pvc. Can you tell me how critical is it the the bottom be perfectly level? Thank you for any advice you can give me.


  35. Hi Peter,
    Since the grid system is a suction system it doesn’t care if it is level or not. So you will not have any problem.

  36. I plan to build a pond that is 5’w x 7’L x 4’ deep. What gph pump size do you recommend for this system to work for the dimensions I provided?

  37. I live in Portland, OR and would like to know of someone in my area who is knowledgable about designing and building a smallish (maybe 1000 gallon size) pond using this type of filtration.

  38. Do you make the grid as a manifold where there are no ends so you have even suction throughout the system or is this not an issue? I would think if it is plumbed with suction at one end of the System you would not get equal suction at either end?

  39. Hi Peter,

    I have built them both ways and it doesn’t make any difference. It might make a difference if the system is too small for the pond that it is in. I always like to make it as large as possible which allows the system to balance it self out and keep everything working correctly.


  40. I have a pond 60 x 40 x 6 foot deep for most of it (there is a shelf 3 feet deep for 4 feet of the 60 foot length). I also have approximately 200 koi 12-18″ long. Looking at adding this to get rid of the green. Would a system 30 x 40 be large enough? And would I need to pump 60,000 gallons an hour through it or am I misunderstanding that? What size pipe to use? Thank you.

  41. Hi Andrew,

    That is a good size pond and a 30 x 40 would sure help. Your pond is roughly 60000 gallons. At that size 15000 gallons per hour would work. Will it get rid of the green water? The answer is yes and no. It will but it won’t happen right away. I would want to use 4 inch for the main feed to the pump and 2″ for the fingers.


  42. Very interesting idea, Mike, based on good principles. My question is about blanketweed. When I’ve had a clear pond (traditional purchased filter & UV lamp) I’ve always had a problem with blanketweed growing. Don’t you get this problem? I’m in the UK, the pond is about 10′ x 6′ with about 6 goldfish – & loads of frogs 🙂

  43. Hi Jackie,

    I don’t get any blanketweed or string algae at all. The first few years I had a UV light on it because it would turn green in late winter but after the pond turned five you can’t turn the water green. I haven’t had a UV on it for 22 years. It hasn’t had any algae of any kind for over 20 years. I am sure that the under gravel filter has something to do with that and also the age of the pond.


  44. Hi Mike. I’m from South Africa, and find your article very helpful. Please correct me. Im busy converting a 9mx5mx1.8m pool to a koi pond. So I will use an undergravel suction system with 2 centre lines of 50mm and arms of 50mm with 3x5mm holes every 300mm in the main as well in the arms. My only concern is the gravel.The 19-25mm crushed stone that we get here is the closest I can get, as pebble is too expensive for such a large area. Do you agree to my measurements and option for gravel? Will a .75kw swimming pool pump do the job for me?

  45. Hi Schalk,

    Your math is fine. I would want to stay away from crushed gravel as that will compact too tightly and clog up. So I would want to use something else even if the size had to change.


  46. Hi Mike,thanks for your reply,about the gravel,will a bigger type of crushed stone work better then if I can’t use 25mm round pebble stone,let say about 50 to 80mm crushed rock

  47. Hi Mike. Regarding my gravel problem – will a bigger crushed rock layer work if I can’t use the round pebble let rock option? Size 50-80 mm.

  48. Hi Mike. I’ve got hold of a pebble mix. Please tell me if this will work, size vary from 10mm to about 80mm.
    Or must I put the big pebbles first then the medium size and then the smaller gravel on top? I wish I could send you a picture.

    1. Hi Schalk,

      That is quite a range from 3/8inch to 3 inch. As long as it is somewhat round it should would. I would want to keep as much as the large 80 mm out of the pond as the larger rock has far less surface area and the gaps between the rocks get too large. But with the smaller pea gravel it will tend to fill in the larger gaps.


  49. Hi Mike,

    I am building a raised formal pond 25 ft long, 5 ft wide, and only 2 ft deep (~1,800 gal) for a heavy Koi load. I realize the pond is too shallow but I am boxed in by the local building codes. I am planning an under gravel suction grid of six 2 inch pipes ( with 3/8 inch holes 6 inches on center) 25 ft long spaced 1 ft apart and manifolded at each end. I was going install 6 inches of 3/4 Inch to 1 inch rounded gravel (3 1/2 inches above the top of the pipe). I will have a waterfall at one end and a skimmer at the other end. The system will be connected to a 4,000 gph external pump and bead filter. Is the pipe big enough for the 25 ft long runs? I was going to connect to the gravel filter at the skimmer end but should I connect in the middle of the 25 ft side for more even suction distribution? Is the gravel too deep? Should there only be 2 inches above the pipe? Will 2,000 gph (half of the suction going to the skimmer) be enough or should I increase the size of the pump? Thank you for all of your help.

  50. Hi Kirk,

    The system sounds just fine. The 2″ of gravel over the pipe is a minimum for the system more will not be a problem. The suction at one end won’t be a problem. But I would look at a larger pump. By the time you have the dynamic head and the static head with the bead filter in the equation I would want to get up to 5000 to 6000 gph. I would probably go with something like close to 6000 gph. The system should work great.


  51. Hi Mike,
    I am very interested to build this system.

    I have 2 small pond side by side connected with small channel act as a waterfall (pond A wall has lower height than pond B). I think of using suction under gravel system in pond A pumped using submersible pump to pond B using pressure under gravel system. The water then return to Pond A by small channel. Is it possible?

    Can you elaborate more about how to build pressure under gravel system? I tried to search on google but no luck so far.



  52. Hi Ardy,

    Yes it is possible to do what you want to do. Lets start with pond B. This is to have a pressure under gravel system. It is built like the suction grid except for the position of the holes in the pressure pipes in the gravel. The holes drilled in the pipe are drilled in the same interval as the suction grid. The holes are drilled so that there are two holes 180 degrees from each other and are positioned so that the water comes out the sides of the pipe parallel to the bottom liner. This pipe is connected to the output of the pump. The pump has to be located in a container under water in Pond A. The under gravel grid in this pond is constructed the same as is outlined in the article. The pipe from this grid goes to the container that has the pump in it. The pipe if run through the wall of the container and not connected to anything. The submersible pump is in this container and the output of this pump is connected to the pipe going to pond B pressure grid. This container is constructed so that the only water that can get in the container comes from the under gravel grid. So when the pump is turned on the water in the container will pump up to Pond B. As this water is removed from the container water will be sucked into the container from the under gravel filter. Yes this system will work.

    1. Hi Mike,
      Thank you for the explanation, it is crystal clear.

      Now with the detail (I’m using metric system) as follow please check if everything is setup properly:

      Pond A = 1.4 * 1.4 * 0.4 meters
      Pond B = 1.7 * 0.5 * 0.5 meters

      I am using pump lifetech spb 613 with capacity 13,500 liter per hour. Inlet and outlet diameter is for 1-1/2″ pipe size.

      I am planning to use 1-1/2″ for main pipes and 1-1/4″ for branch for both system on both ponds.

      Because of limited height of the pond, I am planning to have only 2″ depth of rock/gravel on top of the pipes.

      Please help to assess for pump capacity vs pipe size.

      Oh and 1 more question, for main pipes do we drill the holes with the same principle as in the branch for each of the system?

      If I want to include picture or design for further discussion, can you email me your email address?



  53. Hi Ardy,

    The pump you have will pump about 3600 gph if the head is low. With the 1.5 inch pipe the head pressure will be raied some which would be about right for the two ponds. The 2 inches of gravel over the pipes is correct. As far as drilling holes in the main line is determined by how close the holes in the branch lines are to the main line. If they are 6 inches or less from the main line no holes in main line if further then holes in the main line. If holes are drilled in main line then they are drilled in the same pattern as the branch lines.


    1. Hi Mike,
      Thanks for the feedback, i will use my current pump for this setup with 1.5″ as the main pipes and 1.25″ as the branch pipes.

      The branch pipes is about 150cm (75cm for each side) and with holes drilled every 30cm it would left me 15cm at the end of the branch pipe. Do i need to drill holes in the bottom of the cap at the end of the branch pipe?

      I am planning to use round rock with size option of 1-2cm, 4-5cm and 5-7cm. Which size option is suitable for my pond? Thanks.


  54. Hi Mike.

    I started to build a 2000 gallons pond, and I have a few questions!
    Can you tell me what size I need to use for the main pipe and fingers? I will need to know the distance between the fingers and the distance between the drilled hole.
    Thank you.

  55. Hi Raz,

    With 2000 gallon pond I would like to see at least 2000 gph and would like to see more. So I would use a 2″ line for the main and 1.5 for the fingers. The distance between fingers is 1 ft. and the distance between sets of holes is also 1 ft.


  56. Thanks for the article.
    I never thought I would find a simple way to keep the water clear. I do have a couple of questions. The project I am doing right now is a 16 x 40 koi pond. I would like to use my current swimming pool pump for the pumping aspect. Is this possible? If not what pump would you suggest? Also on the pressurized undergravel system would this work under a bog? I really like the bogs and hope to incorporate it. Also is there a formula for the sizing of the pipes? I only want to do this one time. Thanks for any input you can give.

  57. Mike,

    I am so confused on exactly how the under-gravel grid is physically hooked into the piping from the pond, through the epdm, to the pump. Is it, or can it, be tied into the Skimmer or does it have to be independent to the pump? Please explain how this will be tied in without any piping showing please.

    1. Hi Clayton,

      It won’t work very well hooked into the skimmer. The piping from the grid can go thru the liner with either a pipe penetration or a bulkhead fitting. The pipe can also be run behind rocks and out over the top of the liner. It works best when the system is driven by the suction of a pump.


  58. Hi Mike, In your article you stated you used limestone rock for bed? I thought / heard that limestone will not work with fish because of the acidity?

    Thanks Gerard

    1. Hi Gerard,

      Limestone will raise the PH and KH. Which is bad if your normal water is closer to normal. My water comes from limestone wells so the limestone in the pond is not a problem.


  59. Hi Steve,

    Most under gravel systems are run by an external pump. This pump can also be used to run a skimmer and the output of the pump can easily run a waterfall. Most ponds have skimmers and waterfalls. All but one of the ponds in the pictures have skimmers.


  60. So I will be building a fairly large koi pond soon and trying to figure out the best filtration system. Ive read through a lot of the comments and questions. I’m still slightly confused on the route of the water from the under gravel intakes and what will actually be doing the filtration. I plan to use an external pump but not sure how to make all this work. Could you possibly draw out and upload a diagram on the flow path and parts of this filtration system? Thank you very much.

  61. Hi Hoodie,

    The under gravel grid is connected to the intake on the pump. This way when the pump is running it sucks water thru the gravel and then thru the grid. What does the filtration is the gravel that the water has to flow thru.


    1. Hi Mike, you’re article is awesome I was truly at my wits end trying to figure out how to help my algae ridden pond I try to stay away from harsh chemicals but I finally found you,
      I have a 9 x7 ft 3 1/2 deep pond with 5 koi I believe I’m approx 1000 gal with 1200 gph pump and 1200-2000 pressure filter. If I understand correctly I can use a 2” middle pipe with 1.5 ‘ fingers with 2’ of 3/4 round gravel? Please correct me if I’m wrong. Now about holes I’m confused on I have seen someone do slits down to the middle of pipe instead of holes? Thank you

      1. Hi Jenn,

        Am I correct in thinking that you have a submersible pump. If that is the case it can be used but not easily. If all you are having is a problem with algae. I assume you are referring to string algae and not green water. If this is the case then I will tell you how I control string algae. There is a bacteria on the market made by Fritz industries called Pond 360. They make this in powder and liquid. This only works if you use the liquid. Treat it per the instructions and then with the second dose that is normally 2 weeks after the first dose add a half dose of either Algaefix or Algae Cleanout. Within a couple of days most if not all the algae will be gone.


        1. Thank you I will look into those products, so my other question is would the under gravel filter system be a good addition to my pond, yes I have a inlet submersible pump but if needed I can get a different one.

          1. Hi Jenn,

            Yes it will help your pond. A submersible pump can be used but it is not easy to make it work that is the big reason that I recommend an external pump.


          2. Update: we did it, yayyyy cleaned out all the sludge and made your UGF systems and it works awesome, I have to admit I was scared that our pump wasn’t strong enough or wouldn’t produce enough flow and I am here to say it works even better, Thank you so much we even ordered a couple Algae hi fine banded shark to add our pack, I’m so happy thanks again!

  62. I was thinking of building a semi formal koi pond of around 2500 gallons using a combination of under gravel and bog filter. How do you think that system would work ? Would it be over kill for a moderate fish load?

  63. Hi Fred,

    It will work great. There are very few koi ponds with a moderate fish load. They may start that way but they tend to grow way past that point. If the system is correctly designed it will be able to handle what ever you throw at it.


    1. Mike,
      Thanks now if you don’t mind could you help me with layout and pipe, pump size I would be very thankful. The pond will be approx. 5 ft x 15 ft with a depth of 4.5 ft. Bog will be 1.5 x 15 ft.
      Again thanks so much for the help


  64. Hi Fred,

    I would use 2″ pipe down the center with 2″ branches every ft that go out to the outside edge of the pond. With the bog I would use one 2″ pipe along the edge of the bog with 2″ legs every ft. The pump size I would use is about 5000 gph on this size pond. You could get by with something smaller if you need to but I would not.


  65. the holes in the suction pipe is there one hole every 6 inches or do you drill through the pipe and have 2 holes across from each other the holes for the suction pipe on top or on bottom or on the sides when you put the rock over the pipe I’m using 2 inch pipe at the bottom of a 40 by 40 pond

    1. Hi Lyle,

      The holes should be every ft. These holes should be a set of three holes with one on the bottom of the pipe and one hole 60 degrees from the bottom hole in each direction. With your 2″ pipe one hole would be at the bottom and then one hole 1.5 inches from the center of the bottom hole going in around the pipe so that you end up with three holes.


  66. HI Mike,
    I am building a concrete pond that is (inside dimensions) 6’8″ x 8’8″ x 5′ deep. approximately 2,100 gal. After reading all the info I’m looking to build the gravel filter.
    One 2″ main pipe with 1.5″ pipe fingers? (fingers 1′ apart?)
    Holes size 3/8″ diameter?
    Hole spacing on 2″ main line, 1′ apart? 3-holes (bottom and 60degrees from bottom)
    Hole spacing on 1.5″ fingers, 1′ apart? 3-holes (bottom and 60degrees from bottom)
    Pump size, able to pump a minimum of 2500 gph (going to have approximately 5′ of head raise to a water fall, I will allow for the head loss)
    What type of rock bed, 3/4″ to 1″ round rock ? or 3/4″ crush rock? ( 2″ to 3″ cover over pipes?)

    Am I on the right track?
    I would like to have your experienced input so I can try to do this right the first time.
    Thank you,

  67. Hi Eric,

    The main line does not need any holes but you can put them in. If I was going to put holes on the main line the holes would be centered between the branch lines. You are right on everything else except the crushed rock. I have never used crushed rock before and I can see many reason why it would not work.


    1. Hi Mike,
      Thank you for getting back to me. The concrete part is done. Going to get started on the rest.
      Thank you again for all the great info.

  68. Very interesting article. Thanks for taking the time to write it and for answering all these questions. I “converted” my 10,000 gallon lagoon shape swimming pool into a tilapia pond last year. Since I wasn’t sure this was going to be a permanent change, I haven’t modified the pool filtration system much. But some of my 60 tilapia I bought last year decided to reproduce, so now I have over 200 out there, from 1/2″ to 12″. I have kept the water clear by filling an old very big ice chest with barley straw and a thick bed of floating expanded clay pebbles on top, where I have a lush crop of alfalfa going. That tank is kept full of flowing water from the pool. I also have some marsh plants on the steps and love seat, pulling as much nitrogen out of the system as I can. The pool has 2 suction drains at the bottom in the 8′ deep end. I’m thinking I can put a few feet of 1″ round granite pebbles in the entire “bowl” that is the deep end to create a filter bed, and that it might work just fine without a PVC grid with drilled holes etc. There would be suction via the drains under the gravel, and I think there would be good water flow through the rocks. I could supplement the drain flow by burying a circular grid under the gravel in the deep end following your basic guidelines, and provide suction to that grid by connecting it to the unused hose/side port for the pool sweep. What do you think?

  69. Hi Jim,

    I don’t think it will work without the grid. With the drains the area right above would be fine but when you get away from there the gravel will clog up. There is no reason that the grid can’t be circular grid.


    1. Mike, is more better in this design. Could you place the fingers off the main line closer together and add the rocks to 2” above the pipes. It seems to me the large areas between the pipes don’t provide any filtration since the suction is essentially coming from an area above the pipes/fingers with the holes drilled. And since the system is self adjusting with the suction being moved to wherever it is needed, wouldn’t more pipes and more downward facing holes be better? One final question, are any of the possible rock/media too sharp for koi? Is there a danger of them being cut on or scraping on the rocks?

  70. Hi Sean,

    I have no idea if moving the piping closer together would change anything as I have not done this before. I know the way I designed works and have never had any build up anywhere in the system. The 2 inches of rock above the piping is needed. The holes in the piping is on the bottom half of the pipe. The rock that is used is smooth in nature and is not crushed gravel. Crushed gravel could be a problem in that it will pack together tightly and could clog up. Could the koi be hurt with this type of rock? Anything is possible but with the smooth rock it can’t happen.


  71. Mike,
    I’m amazed by the detail in your answers. I don’t have a question I just wanted you to know, I’m building a swimming pond for my 30 year old developmentally disabled son. My wife and I will enjoy it with him of course. There will be a biofilter pond, stream and waterfall. I have gleaned many ideas from these posts and your answers here have made my design work easier. Thank you

  72. I came across your article and was intrigued to inquire what I should do seeing I have a 70×60 x average 4′ deep pond or abut 120,000 gallons. I’m in the process of some sort of separation to the pond as it adjoins to my neighbors who has just as large a pond (we dug this together when we were building our homes but he not interested in investing in keeping it “clean”),

    Currently it was lined about 20′ around the circumference with small limestone “boulder” about 10″ set every 2′ square to hold the liner in place. We have a lot of tress in the area that when the wind blows we get a lot of leaves that end up sinking causing a lot of decay and nutrients. I have about 300 small Koi about 6-8″ that have over wintered (here it gets 30-40 below and the pond freezes over to a foot thick). Currently I have an aerator and a pump that cares water over a larger rock as a water fall. The next step planned is a pond large skimmer so we can collect the leaves.

    Over the past number of years I’ve used a number of products to keep the allergy down including muck away but I can’t keep up. I’m thinking of planting a number of various plants such as water lilies and grasses as one way to help but also to protect the fish form predator’s such as eagles, cormorants and hawks (even throwing in a few culverts pieces here and there for protection).

    With a pond this size I’m wondering if your filtration system is the way to go. If you’d like I can send you some pictures to get an idea of what we’re dealing with. Also as I’m one who owns a business I would expect a cost to this sort of expertise but if your willing to help all the better. Either way let me know and thanks for your time.

  73. I know this might sound crazy, but I’m attempting to build a formal 8×10 4’ swimming pond in my back yard with a waterfall… I’d like to use your system to filter the water between the waterfall and the formal swimming pond which will have concrete retaining walls with a pond liner… I was thinking of using 2” schedule 40 on the sides of the pond with the holes drilled every 4” at sixty degree angles with about 3” of 1” gravel on the sides covered with some flat stone with gaps to make the bottom of the surface smooth… So, in effect, I’d have the pond pump with the gravel system on one end of the pond and then I was thinking about using another filter pump on the other end of the pond with a skimmer… Do you think this would work or would I just be wasting my time? Also, would I need to make certain the two pumps flow at the same rate?

  74. I forgot to mention I also am planning on using a screen between the waterfall and pond to collect debris like leaves where I could just clean that out real quick…

    1. Deanna,

      I am sorry but I am not licensed in swimming pools so I can not tell you if it would work or not.


  75. Hi Mike,
    Great article and thanks for your detailed and patient responses.
    1) if suction/submersible pump is totally contained within the pond, will the pump output be crystal clear or will it still contain some waste particles?
    2) does the under gravel grid system remove nitrates? If not, how should nitrates be controlled?
    Thanks, William

    1. Hi William,

      If the submersible pump is in a container that can only get water from the grid the water contain very little if any particles.
      No it can not remove nitrates as there is no bacteria to remove these. That is what plants do.


  76. Hi Mike,
    Thanks for the informative post.
    I live in Chicago (East Rogers park, by the lake) and am lucky enough to live in a unique area that’s allowed me to construct a fairly large pond with waterfall.
    I don’t know if you ever make it my way. However, if you do it would be a pleasure to show you what I’ve constructed.
    As that’s probably not the case, I hope I can pick your brain a bit.

    A) Since algae is the biggest pain in the butt, and that’s what I’d implement your design, why don’t the drilled holes just clog up in your system?
    B) If I’m using a pump to drive the waterfall, do you suggest an additional external pump to drive your system?
    C) Does your system basically suck debris from the gravel, get skimmed externally, then dump that filtered water into the water wall reservoir, before returning into the main pond?
    D) If I used black PVC, without covering it with gravel would the holes clog as mentioned above?
    E) Do you think I could pre-construct your system, then just lower it into the pond, so I wouldn’t need to drain it?

    I hope to hear back from you.
    Thanks in Advance,

  77. Hi James,

    Thank you for the invitation but doing the season I am so busy I usually don’t have time to visit very many ponds. But July 26 my pond is on the Midwest Pond and Koi Society pond tour. Maps are available all over the chicago land area.

    A) Algae is not going to get through the rock to get to the pump so it can’t clog it up.

    B) No the output from the pump can be used to do anything you want it to do.

    C) I am not sure that I understand what is question is. The gravel in the system is there to grow bacteria to remove biological waste from the water. It will stop larger debris from getting into the system but it still will sit on top of the gravel. The water that is pulled thru the system can be used for anything you want it to do.

    D) Yes without the gravel the holes could get clogged up with just about anything.

    E) Yes you can pre-construct the piping and lower it into the pond. Yes you could pour the gravel thru the water on top of the piping. But you would still have to empty the pond to get rid of the dirt from the rock unless that was perfectly clean before you put it in the pond.


  78. Hi Again Mike,

    Thank you for such an expedient reply & sorry to bother you again.
    I’m just a little bit confused about (E).
    As long as I clean / vacuum the liner & wash the rock to be added in a wheelbarrow, before adding it to the pond, would that do the trick?
    Lastly, can you be a bit more specific on the type of gravel/rock to be used? Black gravel, river rock, pea gravel?
    If I do this, the last thing I want is to cover the system with the wrong material, then have to redo it!

    Thanks Again.
    I hope not to bother you again…lol

  79. Hi James,

    The liner won’t be much problem other then dirt from walking on it. The rock is not easily cleaned outside the pond. It can be done.

    The gravel I like to use is 3/4 to 1.5 inches in diameter round none limestone rock. It goes by a lot of different names. A good place that you would be able to get it would be Lurvey’s in Des Plaines. I don’t know if they have it in bags but they have it in bulk.


  80. Hi Mike

    We’ve been struggling for years with our waterfall pump constantly clogging up with algae & crud.
    My husband has tried ‘MacGyver-ing’ more prefiltering solutions than you can imagine. (Or maybe you can!)
    If you think your system would work, we’ll give it a try!
    Our pond is approximately 12′ long x 7′ wide x ~ 4′ deep at the deep waterfall end to ~ 18″ at the shallow end.
    Could you please tell us what the minimum size PVC pipe could be used?


    1. Hi Sue,

      I would guess that you have about 1200 gallons in the pond. You didn’t say this but I would guess that your pump is in the pond. If this is the case I would think about putting a skimmer on the pond with the pump in the skimmer and let the skimmer do the job of getting the algae and crud.


  81. Hello Mike,

    I have a pond that is about 2,500 gallons. I am moving the pond in the fall to a new location and i’m upgrading it to 4-5 thousand gallons. I am going to use a PondKeeper 5000 for the filtration and a small pump for the skimmer, using the skimmer water return for the waterfall.

    I have three questions.

    I want to use the grid system you have in the picture above but wanted to know if my idea of taking it a step further would be a good one, or a bad one. I was going to install plastic grid locking tiles (they resemble an underwater filtration system) over the pipe before installing the rocks. Would this be okay to do? Is there a downside to this that you know of?

    My second question is. What is the formula for drilling the correct amount of holes or even pipe size for that matter? Can I just use 2″ pipe throughout, drilling holes 6 inches apart and it will work itself out?

    My third question is water return. Putting 70 gallons a minute back into my pond seems like it will create a pretty hefty current. I haven’t used anything to this degree and i’m unsure if I should just have three or four returns throughout the pond. Or is there a better way of handling this?

    Any help would be much appreciated.

  82. Hi Ray,

    The GC Tek system is an excellent system. You might want to look at other GC Tek systems. I personally prefer the Alpha One LH systems as they do great job and don’t need 2 speed pump.

    You can use a couple of jets or this can run over a waterfall.

    If you are using 2″ pipe which is good then if you add up all the surface area all the holes you want that value to be anywhere from 5 to 10 times the surface area of a cross section of the pipe. The surface area of the cross section of 2″ pipe is 3.14 sq. inches.

    As far as the grid over the pipe I can see that being a problem and not needed. Just put the drilled holes at the bottom half of the pipe.


  83. Good evening Mike. Could you give me any specific details on using an airlift to run this SG? My goal is to use 2” pipe down the center with 2” branches every foot at the bottom of the pond. Cover with 3-4” of 3/4”-1” round river rock. I would like To use an air lift going down to lift the water from the bottom and up and out in the pond. The outlet holes would be drilled in the upper level of the pond on a plant shelf. Kind of acting as a bog within the pond. I’m still trying to figure out how to connect the bottom of the airlift where the compression chamber is to the pipe of the SG. I was planning on building the airlift based off the original airlift for your pond by Aquatechnobel off youtube. I would appreciate any and all feedback from you. Thank you for this article on the SG. I’m so excited to get this built and have a minimal maintenance pond. Thank you, Mary

  84. Hi Mary,

    Air lift systems can work very well with an under gravel filter. The system doesn’t care how the water is moving thru the system. I worked with a person I know in Mn to design under gravel filters in his swim pond that can run with either air lifts or pumps. That was about 6 years ago and the system still works perfectly. If you have a problem with using an airlift with the under gravel filter it is usually a problem with the air lift.


    1. Hi Mike

      Are you avail for consulting of an exciting new shallow pond? This is for a new home on Martha’s Vineyard. It will be approximately 120ft long by 30 wide…..we will use white gravel with your suction pipe system beneath the stone. All water will be piped to filtration system with chlorinator as our hope is for crystal clear water.

  85. Hi Mike,
    I clean my rock by spraying it with water in a well perforated (on the bottom and lower 1/3 ) of a Home Depot bucket over my lawn. I did it for 2 tons of 3/4″ – 1″ rock with 3 buckets.

    1. That is one way to get them clean. I usually clean the gravel in the pond once it is in place. I take the hose and wash the gravel down and pump the dirty water out. The bucket method does work.


  86. Hi Mike, nice artical about pond filteration.
    Just wondering, isn’t 1 inch gravel can possible clog 3/8 inch holes?
    Is it better use 2-3 inch gravel on 1st layer before 1 inch gravel as 2nd layer?

    I’m planning pond size 8ft x 9ft x 4.5ft.
    Planning grid pipe size 2 inch.

    Thank you!

  87. Hi Tiago,

    I don’t know if 2 to 3″ gravel would work very well or not because I have never used it. I know the 1 to 2″ works great with the number of holes drilled from experience. Any change I don’t know how it would work. I have done one pond with pea gravel and smaller holes and it works.


  88. Hello Mike, thank you for all the great info. I have two ponds that hold water but need quite a bit of work. My goal is to fix them up and have clear water in them. I am down near Austin Tx. Do you know of anyone down here that can help me implement your system in my ponds? Also, do you have any other resources like videos or a type of guide that help me out in case I decide to tackle this my self?

    Thank you,

  89. Hi Felipe,

    I am sorry but I don’t know of any in the Austin area. I do know of someone in the Houston area that has built some of these ponds that work perfectly but that doesn’t help. The two articles in Pond Trade Magazine will give you all the information that you would need to do it. What is not in the articles is in all the questions that I have answered.


  90. Hi I’m looking at building a 15x25x5 1/2 swim pond with raised bog system with two waterfalls to the side. I’m planning on using the uplift system in both the swim area (15×25)and bog area 8/25. I’m hoping the uplift system in bog area will push water up and then spill over to both waterfalls going into swim area. There will be skimmer in swim area. Wondering what your thoughts are. I have some sketches on my project but can’t figure out how to put on here lol. Thankyou.

  91. Matt,

    Am I correct in thinking that by uplift systems you are talking about air lifts. Am I correct in thinking that the lift system is going to push water to the bog system and then a lift system is going to raise the water to a waterfall. If you are talking about air lifts they can raise water slightly but only a couple of inches and the flow drops way off. A skimmer you can run off an air lift system as long as you don’t try to lift the water above the pond level. As long as you are trying to raise the water much with air lifts they can work.


  92. Hey mike Thxs for response.
    I think im using wrong terminology lol.
    I think it’s called uplift system for bogs. Where the pump pushes water up threw the bog.

  93. Hi Matt,

    Yes it makes a big difference. The terminology that you are looking for is up flow meaning the water is feed in at the bottom and then flows up thru the bog to get out the top. So the system you are now talking about can work just fine with the right amount of water being pumped from the right areas to the right areas.


  94. Ok Thankyou.
    Can I put upflow system in swim area? Wanting to cover bottom with sand or rock!
    There would be no plant’s in the swim area. Just thought it would be good to have movement. Thanks again.

  95. Have you ever put sand on the bottom. If so how did it turn out. Little skeptical on doing it
    But the fam jam loves the look so figure I’ll try it out.

  96. What type of pond vault do you recommend that can be “sealed” to use with a submersible filter? Do you mean seal it manually with caulk or liner then and add an additional bulkhead for the intake pipe? Thank you.

    1. Hi Leslie,

      I have used the Savio pump vault before. It is not too back to seal but any more I would use a submersible pump that has a threaded intake on it which usually means a mag drive pump. This means that small rocks will destroy the impeller on the pump so that is why I attach a leaf basket designed for an external pump to the intake of the pump and then attach the piping from the under gravel filter to the intake on the leaf basket. No rocks are going to get thru the leaf basket but the leaf basket may fill up at some time. This design allows the under gravel filter to work the same way it does with an external pump.


  97. Hello Mike I’m building a turtle pond of about 2000 gallons with up to 6 turtles and some fish. What size pipe should the main line and arms be? And how many holes and how far apart would you recommend?

  98. Hi Joshua,

    There is no set answer to your question. The size of the piping and holes is more depend it on how much water you are wanting to pump. A few questions. Are you having a waterfall or stream and how large. Are you going to have a skimmer? Are you having any other filtration. What kind of pump are you using?


    1. Hello Mike,
      Yes I have the SAVIO 16inch waterfall filterweir skimmer. As well as a water fall with filter mats for biological filtration at the bottom that will be around 2-3 feet tall fed by a 2100 gph pump. And I have a 1000 gph pump for the gravel filter.

      Thanks Joshua

      1. Hi Joshua,

        With you only moving a 1000 gph thru the under gravel filter I would think about using 1″ piping and drilling 1/4″ holes. I assume that you are going to use the Savio skimmer with the 2100 gph pump. I would think about using the 8″ weir in the skimmer.


  99. Mike,
    I have read both of your articles and every comment after both multiple times. I love them and have been taking notes along the way. Do you find a difference or have a preference when designing a UGF to use 3-way tee fittings, alternating the branch directions, or 4-way cross fittings with directly opposing branches?
    I have a 300-gallon Rubbermaid stock tank pond that is 62 inches by 55 inches on the floor that I’d like to incorporate a UGF and a 50-gallon tank that is 46 inches by 25.5inches on its own floor above the pond to serve as an up-flow bog filter. I have on hand a 4,500 GPH submersible pond pump with a 1.5” inlet and a 2” outlet. I’d like to use the single pump to run the UGF and the bog. I think I understand from my reading that I should match the inlet and outlet pipes on the pump to preserve the engineered pumping rates. The head height from the pump vault to the outlet pipes on the floor of the bog is less than 3 feet. I read in the comments and articles about the branches being a smaller diameter pipe than the main line. Can you briefly or simply explain the reasoning for this? I get myself too deep into the weeds when trying to research fluid dynamics on my own.
    You also mentioned that the smaller the size of the pond, the more often the entire volume of water must be turned over. Is there a formula that you use for this to work properly?

    Very respectfully,
    Dennis E.

  100. Hi Dennis E.

    There is a lot here in this question and I will try to answer all the questions. The first question is crosses or tees and the answer is that it doesn’t mater as long as they are spaced properly. On your 300 hundred stock tank pond I would rethink the 4500 gph pump. That could turn your system over every 4 minutes which is way too fast. I would prefer to keep the pump about around 1200 gph. As far as matching the pipe size to the pump intakes and outputs is incorrect. You want the inlet piping to have the same or less head pressure as the output. The only reason for the branches to be smaller piping then main line is that they are moving less water and the friction can help balance the flow thru the system. The guide lines that I use on pond turnover goes as follows. Less then 500 hundred gallons I want to turn it over about 4 times per hour, 500 to 2000 gallons turn over 2 times per hour, 2000 to 20,000 gallons about once per hour. These are the guidelines that I personally use as minimums.


  101. Hi Mike
    I like the idea of under gravel filtration especially if it does not require frequent cleaning. Can the system remain running when the Koi are spawning and when there are fry in the pond? I am just concerned that the little ones getting sucked in.

  102. Hi Abdullah,

    Anything is possible but not very likely because there is not that much suction in one small point at the surface of the gravel. Secondly the koi normally spawn near or at the surface. But out of the thousands of eggs some might go thru the system. I do know that some of the eggs do survive going thru my undergravel system in my own pond. An egg once or twice every four or five years survive. But I don’t know how many go thru the system so I don’t know what percentage make it.


  103. Hello Mike. Great article. On an air lift method does the bottom of the vertical pipe go deeper than the grid or is it level?

  104. Hi Mike
    Great article but not sure if this has been asked, so here goes.
    Can this work with gravity fed filter as well?
    I’m kind of guessing yes, but seeing if you have come across this.

    Kind regards Craig

  105. Hi Craig,

    I am not sure I know what you mean by a gravity fed filter. If you are saying that the water flows into the filter by gravity and then is pumped out of the filter.If that is the case there is no reason why it would not work.


  106. Hi.
    I am going to redo my 16x8x3 pond so it is 4 Ft deep with a bottom grid system.
    I saw that the “Ponddigger” recomended having an air ring around the edges of the pond. What is your preferred method to aerate the pond and create the updraft for sinking leaves etc.? Is a center air disk just as good?
    My pond does have a skimmer and TPR jets to swirl the top and redirect floating things to the skimmer.

    I am wondering if it makes sense to have a dual suction / pushing water in that size pond.
    Pushing water back will push any lodged gunk to move upward and probably get scooped up better. I am going to have a min 6500GPH – pump with split output – 1. return to the the stream 2. Returning water to the TPRs.

    1. Hi ZVI

      With an under gravel filter system you get rid of a lot of the dead areas in your pond as water is being moved over the entire grid. I personally prefer to use air to get movement in dead areas. You have to remember that air bubbles below the surface do only one thing and that is to move water to the surface where it can absorb Oxygen. So I prefer to have air bubbling in area with little water movement. Air is a very cheap way to move a lot of water. I also prefer to put the air where it is going to not disturb the surface in the viewing area.


  107. Hi:
    I going to redo my 16x8x3 so it is 4 Ft deep with a bottom grid system.
    I saw that the “Ponddigger” recomended having an air ring around the edges of the pond …
    what is your preferred method to aerate the pond and create the updraft for sinking leaf etc.?
    is a center air disk is just as good ?
    My pond does have a skimmer and TPR jets to swirl the top and redirect floating things to the skimmer.

    I a wandering if it makes sense to have a dual (suction / pushing water ) in that size pond ?
    pushing water back will push any lodged gunk to move upward and probably scooped up better.
    I am going to have a min 6500GPH – pump with split output – 1. return to the the stream 2. Returning water to the TPRs.

    Any comments .


  108. leonard merriman

    Where can one go to get step by step on how to hook such a system up . No one shows the mechanical aspect of the gravel system. So I am clueless how to hook it up

  109. Hi Leonard,

    If you read both articles and all the questions and answers I think you will find all the answers to all your questions. If not then post your exact question and I would be happy to answer it.


  110. Hi Mike, this is great. I have a 20 x10, 3 feet deep pond. If I understand, 3 inch main pipe, 1.5 inch arms. 3/8 hole in arms every 3 inches, facing down,holes in main pipe as well. What is that spacing on the arms? (ie every foot?)

    I have a 1.5 HP (70 gpm) pump that pulls to my filter and feeds my waterfall (about 4 feet of lift) and a separate skimmer pump.

    currently I have an single 3 in pipe running length down the middle with holes, on a bare bottom, but just can’t seem to keep it clear. I am thinking I can just add arms and rock.

  111. Hi Greg,

    I would guess that you have somewhere between 3000 to 4600 gallons of water depending on how the pond is shaped. One thing I would want to rethink is the pump that is running the system. 1.5 HP pump that is moving 4200 gph is going to cost a lot to run when it can be done with about a quarter hp pump. At that rate of flow I would think about using 2″ piping for the main and to the pump. The spacing on the piping is 3 holes every ft. The arms off the main line should be spaced every 1 ft. This will clear the water unless a lot of outside debri is going into the water.


  112. Hello
    I have read every question and answer so I hope I have an understanding.
    I am in the planing stages of a new pond as my Koi have outgrown what the are in now.
    Pond will be 16x 20 3 foot deep. We will include a Bog filter and wanting to have the grid
    feed the bog.
    Separate skimmer to Waterfall filter.
    I am thinking of using 2″ mane with 2″ fingers as described with 3/4 too 1 1/2 round river rock with 2 too 3 inches above grid.
    what size pump should I use for the Grid / Bog ?
    Can the grid /bog be run in the winter ? pond will freeze over but kept open with air and pond heater.
    Could I just use a bypass to bypass the Bog and have the water return to the upper levels in the pond during the winter?
    Would be turning off the skimmer and falls during the winter to avoid Ice dams.
    Pump for Gris would be submersible in the pond.

    Thank You


  113. Hi Chris

    Everything you wrote will work just fine. But I would rethink the submersible pump. A submersible pump can be used but it is far more difficult to get it to work correctly. It also becomes far more difficult to service because it will need to be serviced.


    1. Mike
      Thank you for the response. I keep looking for more information on Airlift but I can not find detailed information.
      would the riser for an Airlift need to be bigger than the 2 inch grid pipe? Will there be enough pressure from this to feed the bog? And what size Airpump?
      If I use the Submersible pump I could have it feed the Bog from watertight vault/box installed in the pond at the edge with the grid feeding the vault,kinda like a skimmer vault but only fead from the Grid below water level with the top of the vault above pond level.

      1. Mike

        Once again Thank you. I was able to get with the folks over at Practical Ponds and they supplied me with pictures and drawings of an Airlift that use and they answered all my questions. Looks like an Airlift is supper easy to make and this has made up my mind to use the Airlift . KISS Keep It Simple Stupid.

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