So You Want a Crystal Clear Pond – A (nearly) maintenance-free system for clean, beautiful water

pondfiltration 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.

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.

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

Rock and Gravel

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 a
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.

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

Undergravel Suction on a Small Scale

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.

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.

An undergravel grid installed and ready for gravel.

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 stumble
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.

77 Responses to So You Want a Crystal Clear Pond – A (nearly) maintenance-free system for clean, beautiful water

  1. dominic carone June 11, 2016 at 12:03 AM #

    Great article Mike. Nothing like and under gravel suction grid!

  2. Suzanne July 28, 2016 at 3:48 AM #

    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?

    • Mike White July 23, 2017 at 9:18 AM #

      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.

  3. Lora Lee Gelles July 28, 2016 at 9:21 PM #

    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.

  4. Ed OBrien September 19, 2016 at 9:21 AM #

    In the main suction line are there holes drilled the same as in the legs, or are they left solid?

  5. Edward OBrien September 19, 2016 at 3:53 PM #

    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

    • Lora Lee Gelles September 25, 2016 at 8:56 PM #

      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.

      • Ed OBrien September 27, 2016 at 3:18 PM #

        Thank you sooo much for the info. I will send photos when I am finished!

  6. Karl November 3, 2016 at 7:28 AM #

    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!

    • Mike White July 23, 2017 at 8:57 AM #

      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.


  7. Jeff Higgins March 30, 2017 at 7:20 PM #

    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

    • Mike White July 23, 2017 at 8:43 AM #

      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.


  8. Tim Camp March 31, 2017 at 4:44 PM #

    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.

    • Mike White July 23, 2017 at 8:44 AM #

      Hi Tim, If there is anything I can help you with let me know.


  9. Tymber April 12, 2017 at 12:48 PM #

    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?

    • Mike White July 23, 2017 at 8:37 AM #

      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.


  10. Wayne Allison April 23, 2017 at 8:39 PM #

    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

    • Mike White July 23, 2017 at 8:22 AM #

      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.

  11. Jacob May 25, 2017 at 6:41 AM #


    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?


    • Mike White July 23, 2017 at 8:07 AM #

      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.

  12. marco millet May 26, 2017 at 11:20 AM #

    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.

    • Mike White July 23, 2017 at 7:50 AM #

      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.

  13. Cameron July 4, 2017 at 4:37 AM #

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

    • Mike White July 23, 2017 at 7:43 AM #

      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.


  14. Ben Bowen July 27, 2017 at 5:46 PM #

    Considering just filtration (not aesthetics) is there a clear winner between round rock and a fractured gravel?

    • Mike White July 27, 2017 at 6:34 PM #

      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.

  15. Hamid August 9, 2017 at 10:07 AM #

    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

    • Mike White August 10, 2017 at 4:13 AM #

      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.

  16. Rando August 25, 2017 at 4:21 AM #

    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.

  17. Mike White August 25, 2017 at 5:31 PM #

    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.

    • Rando August 28, 2017 at 10:08 PM #

      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.

      • Mike White August 30, 2017 at 4:04 AM #

        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.

        • Rando September 4, 2017 at 12:38 AM #

          I see. Thanks for clearing that up. So how do you address the neutrally buoyant waste?

  18. Dave November 9, 2017 at 5:15 AM #

    Hi I’m just starting a pond and wonder what size pipe and pump I would need to do a 16×26 foot pond 5 feet deep

  19. Mike White November 9, 2017 at 3:15 PM #

    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.

  20. Dave Myers November 21, 2017 at 5:42 AM #

    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

  21. Jim April 15, 2018 at 1:42 PM #

    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.

    • Mike White April 15, 2018 at 3:13 PM #

      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.

      • Jim April 16, 2018 at 9:25 AM #


        Should I prime and glue the pvc?

      • Glen October 5, 2018 at 4:25 PM #

        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?

        • Mike White October 7, 2018 at 9:58 AM #

          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.


  22. Mike White April 16, 2018 at 12:12 PM #

    Yes I would. I would not want the piping coming apart under that gravel.

  23. Wei Wei Jeang April 22, 2018 at 11:55 AM #

    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!

  24. Mike White April 22, 2018 at 3:15 PM #

    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.

  25. Cliff May 8, 2018 at 4:09 PM #

    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.

  26. Mike White May 13, 2018 at 7:08 PM #

    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.

  27. David Britt May 19, 2018 at 4:42 PM #

    I’m interested in creating a bo similar to this one.

  28. zia nizami May 25, 2018 at 3:00 PM #

    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

  29. Mike White May 26, 2018 at 6:23 AM #

    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.

  30. Robert Mountain June 6, 2018 at 10:03 AM #

    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?

  31. Mike White June 7, 2018 at 4:10 AM #

    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.

  32. Patrick June 8, 2018 at 3:15 AM #

    Does the pump need to run 24 7 or can you cycle it to save electricity?

    • Mike White July 20, 2018 at 12:20 PM #

      As with any filter the pump should run 24/7 to keep the bacteria alive.

  33. Helmut Ernst July 20, 2018 at 4:32 AM #

    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.

  34. Mike White July 20, 2018 at 12:28 PM #

    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.

  35. Raul July 30, 2018 at 5:05 PM #

    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.


  36. Mike White July 30, 2018 at 5:17 PM #

    Raul, the following link will take to a picture of the gravel I would recommend.

    • Raul July 30, 2018 at 5:36 PM #

      Thanks Mike. That is very helpful.

  37. Dietrich August 3, 2018 at 9:57 AM #


    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.

  38. Dietrich August 3, 2018 at 10:00 AM #

    Or does it go back to the pond because the gravel itself is the bio filter?

  39. Mike White August 3, 2018 at 1:02 PM #

    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.

  40. Deb August 10, 2018 at 4:11 AM #

    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

  41. Mike White August 20, 2018 at 5:35 AM #

    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.

  42. Peter Kohm September 3, 2018 at 8:00 PM #

    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.


  43. Mike White September 4, 2018 at 3:46 AM #

    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.

  44. Diane September 23, 2018 at 1:39 AM #

    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?

  45. Jon Congdon September 23, 2018 at 10:59 PM #

    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.

  46. Mike White September 24, 2018 at 5:43 AM #

    Hi Diane,

    I would say that your pond should end up with 650 gallons and I would want to move about 1300 gph.


  47. Peter September 30, 2018 at 2:03 AM #

    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?

  48. Mike White September 30, 2018 at 7:52 AM #

    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.


  49. Andrew October 1, 2018 at 12:16 AM #

    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.

  50. Mike White October 1, 2018 at 8:12 AM #

    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.


  51. Jackie October 18, 2018 at 2:42 AM #

    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 🙂

  52. Mike White October 18, 2018 at 2:59 PM #

    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.


  53. Schalk November 18, 2018 at 2:18 PM #

    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?

  54. Mike White November 18, 2018 at 7:38 PM #

    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.


  55. Schalk December 8, 2018 at 11:58 AM #

    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.

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