So you (Still) Want a Crystal Clear Pond: Revisiting Undergravel Filtration

Published on April 26, 2018

I wrote an article for the May/June 2014 issue of POND Trade magazine under a similar title: “So You Want a Crystal Clear Pond.” If you don’t recall it, check it out in the archives at www.pondtrademag.com before you dive into this update.

undergravel filter pond
This picture shows the completed piping for an undergravel filter. A layer of gravel should extend up to 2 inches above the top of these pipes, which run out of the pond to the suction side of a pump. Fitz’s Fish Ponds LLC built this pond. (Photo courtesy of Mike Hall)

You might notice that my 2014 article generated a few dozen comments and questions from my fellow readers, so I’m happy to take this opportunity to expand on what I wrote four years ago about undergravel filtration. I’ll also address some questions that have since come up.

First, just a quick update — my first undergravel filter is still running great and has not needed to be cleaned in the last 27 years. I’m guessing that the liner will fail before the filter needs to be cleaned or stops working.

Koi Safety Issues

I came across a Facebook post about three weeks ago with a discussion among several well-known koi experts. They were familiar with undergravel aquarium filters and felt that undergravel pond filters would behave in the same way. In other words, the filter could fail or build up toxic areas.

Could this happen in a pond undergravel filter? It’s possible, but not from the waste from the aquatic life present in the pond. It would require a lot of outside debris infiltrating the pond and simply left there to cause a toxic, rotting situation. Of course, this is true with any pond using any type of filter system. An advantage to having an undergravel filter is that debris is generally not an issue, unless it’s so large that it actually prevents the pump from being able to pull water through it. We’re talking about a layer of debris 4 to 6 inches thick. It would certainly be producing poisonous gases before it would get this thick, and you have to assume that the pump would suck it in and bring it up to the surface, where it would rise into the atmosphere before it would harm the aquatic life.

A likely response to this answer is, “That’s not why undergravel aquarium filters fail!” And that is correct. They fail because the gravel or slots in the undergravel bottom plate get clogged up. When the filter is designed as I specified in my last article, it won’t clog up. Water will continue to flow through all areas of the filter, and the filter will continue to work as advertised.

Size Matters

The next logical question is, “Why doesn’t it build up enough debris to clog it up?” Simply put, in a properly constructed system, there should be sufficient space between the gravel so that debris continues to move and break down, eventually exiting though the filter.

  But what about debris that is too large to get into the gravel? This would normally include outside environmental debris, such as leaves or sticks. This material tends to lay on the gravel surface until it is manually removed, or until it breaks down into smaller pieces. It will break down more quickly with water moving around it, rather than just sitting at the bottom of the pond. But it still could take a very long period of time. It is a good idea to have another type of filtration system, such as a skimmer, that is designed to remove floating debris from the pond. I would recommend a secondary filtration system for any outdoor pond, regardless of its primary filter system.

But how closely do you have to follow the directions for building an undergravel filter, which I outlined in the previous article? I can’t answer this one completely, as I have not branched out from my formula very much.

The farthest I have strayed from my proven design is a pond that I built 11 years ago. The homeowner wanted a pond with a completely calm surface. It was for plants primarily, but they wanted some fish, too. To meet these requirements, I designed the system with a submersible pump in a sealed box with an open pipe leading into it from half of the undergravel filter. The output from this pump went to the other half of the undergravel filter as a pressurized filter. The last thing this homeowner wanted was small gravel, and I had my doubts, but we used pea gravel.

An undergravel grid is ready for gravel.

Eight years later, I talked to the homeowner, who had done zero maintenance on the system. Their only complaint was that they occasionally had to dye the water, because the pond was too clear to show off the plants as much as they wanted. I am not crazy about this system for the average homeowner, as it uses a submersible pump, which doesn’t last as long as an external pump. Moreover, this pond doesn’t have a skimmer, so the homeowner has to remove surface debris manually. Most homeowners would not keep up with debris like this. You can see a picture of this pond in my previous article.

Add an Airlift

It may seem hard to believe, but you can filter a large pond using 60 watts of electricity, or even less. This is another huge advantage of undergravel filtration systems. Because the filter is in the pond, it can run without having to lift water above the surface of the pond. This creates the perfect scenario to implement an airlift system. With 60 watts of power operating a good air pump, you can easily move 10,000 gallons of water through the filter.

What other benefits can be expected from this kind of system? You could not have a better system for oxygenating the pond. If designed correctly, the filter should draw water from the entire bottom of the pond and bring it to the surface, where it can absorb oxygen from the air. There should be no dead areas in the bottom of the pond. Other than skimmers and gravity-fed settling chambers, there are very few other filters that can be run by airlift systems.

underground airlift system
An airlift system was integrated by Magnolia Ponds (photo courtesy of Max Taylor).

There are always going to be some who believe that undergravel systems promote the production of hydrogen sulfide. The truth is, this can happen — if the system is designed incorrectly, that is. I have found that a correctly designed system is about as maintenance-free as can be. My own pond requires fewer than 10 minutes of maintenance per month during the spring and summer. It has been running for more than six months now with no maintenance at all, and it will not need maintenance until the hoop house is removed.

Across the United States, there are hundreds of these ponds that were designed correctly and continue to work perfectly. However, there is no such thing as the perfect system. If there was one perfect system, everyone would use it, and we wouldn’t be having this discussion. But all things considered, you can be assured that if implemented per the design instructions, undergravel filtration works very well.

80 thoughts on “So you (Still) Want a Crystal Clear Pond: Revisiting Undergravel Filtration”

  1. Hello! I like the idea of an airlift system in a undergravel filter pond. But how does it work? And how many gallons of air per hour would you need to move 10,000 gallons? I love your work!

  2. Hi Jacob, The way an air lift works is that the rising air will push water to the surface. To move 10,000 gallons of water per hour I would recommend a 60 lpm air pump. It can be done with a 40 liter per minute air pump but everything has to be just right.
    Mike

  3. Hello Mike,
    I’m writing from Frankfurt, Germany.
    I like the system with undergravel filter very much, but do not understand it in detail.
    Are the tubes the filter or do you pump the water into a mechanical filter and then into a biological filter?
    I’m missing the filter !!!
    The most recommended way here would be to have 2-3 110 mm tubes sucking the water from the ground, bing it to the mechanical and then into a biological filter. From there pumped back into the pond.

    1. Hi Wolfgang,
      In an undergravel filter system the pipes used are not the filter but the collection devise used to collect the filtered water. The gravel that the water has to past thru to get to the collection pipes is the filter. Does the pipes do any filtering? The answer yes but are limited in surface area.
      Mike

  4. Mike,

    Thank you for a follow up article. Your first article inspired me to enlarge my koi pond and include an under gravel filter. Do you aerate your pond or does the suction grid remove the gases and circulate the water?

  5. Hi Teri,

    To answer your question yes I do aerate the pond. That is one of the ways I use to help keep the dissolved oxygen level at saturation for the health and well being of the pond and the inhabitants of the pond. With that said the suction grid can circulate water with dissolved gases to the surface.

    Mike

  6. Hi Mike. I have a 20 x 12 Pond about 30 inches deep. I am thinking of using 2 inch PVC pipes spaced 2 feet apart with the three holes every one foot. I just purchased an in-line submersible/above ground 8000 gallons per hour torpedo pump by Cal pumps. This will feed water to my upper level Bog pond which is 5 x 20 x 12” deep 3/8 Pea gravel made as the Nelson water gardens family suggests. in the big pond, should I use a bigger rock Like 1 inch to 3 inch to maybe even 5 inch and only use 2 inches of rock above the pipe to create better water flow? This is because I’m not filtering water down there, I’m using your system basically to collect water and pump it up to the bog and let the bog filter it. Or should I do it as you say with the 3/4 to 1 inch River rock? I’m afraid I may create too much of a filter on the bottom and starve the bog plants of nutrients. I will have a light fish load of 50 to 75 to 100 fantail goldfish.

  7. Hi Sarah,

    First the plants are going to want nitrates and no matter how much filtration you have they are not going to remove nitrates. I would use the 3/4 to 1 inch gravel. The other thing I would do is I would put the 2″ pipe one foot apart and not 2 feet apart. This will keep the water cleaner which will help in not having to clean the bog filter as often.

    Mike

    1. Hey Mike,
      Like Sarah, I also intend on returning water from my under gravel filter through my bog. Should my bog grid approximate my under gravel grid? It seems that would provide the least impeded water flow. Bog is 12” deep with pea gravel and approximately 1/3 the surface area of the overall pond. I’ll also have a slimmer with twin water fall returns.

      1. Hi Ed,

        If all the water from the under gravel filter is going to the bog then the bog grid has to have about the same number of holes as the under gravel has. If the number is too different it can restrict water on the outpur which will cut down the flow on the input.

        Mike

  8. Hello Mike, thank you for writing both these articles!
    I am in the process of designing and building an 18’ x 12’ “natural wading pond”. This pond will consist of a 1’-2’ plant zone and a 3’ wading zone. I would like the bottom of the wading area to house an under gravel filter system. The base there will be 10’ L x 6’ W x 3’ D. With three, 3/8” holes drilled on the bottom, and at 60 degrees on either side of the bottom every 6 inches, I get an area of the holes to be 50 in^2. Would you recommend a 2.5” main line with 2” arms? Could you advise on a pump size for just this portion of the system? There will be another pump in my skimmer that will feed jets to help push the surface debris to the skimmer. This pond will be in the desert and will see summer temps around 110 F occasionally. Do you think the PVC with 2” of gravel on it could handle people walking on it?

    Thank you,
    Anthony

    1. Hi Anthony,
      As far as the size of the pump to be used I can not give you the size that I would use. I would have to know the volume of the pond and set the size of the pump based on that if it is not going to be used to run a waterfall. The 2.5″ pipe can handle a flow up to about 7000 gph. People walking on the pipe won’t hurt it at all. But it won’t be easy for people to walk on gravel on the bottom.
      Mike

  9. Mike,
    Thanks for these articles!
    I’m just starting to design a double pond, one pond for koi and one for plants and a few gold fish. I plan to have them side by side with a total combined volume of 3000 gallons. I want to use airlift pumps. It appears from your articles that I might be able to use under gravel filtering and a combined airlift pump system. Do you see any problems with this setup? I plan to include skimmers to help keep the debris down.
    How heavily stocked is your 27 year old pond with fish and plants? Do you have any issues with planting media getting free in the pond and plugging the gravel filter?
    Thanks for taking the time to answer my questions!
    Dan

  10. Hi Dan,
    Your system should work just fine if the airlifts are setup correctly. My pond is the home of 75 koi. There are also some goldfish and golden Orfes. I also have frogs, trapdoor snails, and crayfish. I do not know if planting media would be a problem. My pond system is a system of two ponds with the water flowing from one pond into another pond and then back. The first pond has water lilies in it with planting media and the second pond has koi and under gravel filter. There are a lot of plants in the second pond but they are planted bare root in the rocks on the edge of the pond.
    Mike

  11. Mike,
    Thanks for the response. Your ponds sound very impressive! That’s a lot of koi.
    So only your second pond (the koi pond) has an under gravel filter? Do the ponds have a secondary or backup filter system?
    Have you posted photos of your pond anywhere? I would very much like to see what it looks like.
    Thanks,
    Dan

  12. Hi Dan,

    Yes the pond has a couple of other filters on it. The water from the undergravel filter flows to a lava rock filter that has 3,000 lbs of lava rock in it. There is also a homemade box filter with PVC ribbon and filter pads in it. There are no pictures of my pond on the internet.

    Mike

  13. Mike,
    I’m new to building ponds and have read both your articles. The idea of a large pond with a low maintenance filtration system sounds like a dream come true, but I would like more details on how to calculate the size of the pump I would need for my pond. I would also like to know more about how to physically assemble this system. Can you point me towards some helpful resources?

  14. Hi Adam,

    The size of the pump is more dependent on the volume of the pond to determine the smallest amount of flow that is needed. I use the following formula. Less than 1000 gallon pond at least a 2000 gph pump, 1000 to 4000 gallon pond a 4000 gph pump, 4000 to 10,000 gallon pond a 5000 gph pump and 10,000+ pond a bigger than 5000 gph pump.
    It is easiest to assembly right in the pond when building the pond. Pipes should be drilled ahead of time.

    Mike

  15. Hi Mike,
    I am constructing a raised 25 foot long by 5 foot wide by 2 foot deep (~1,800 gal) koi pond with a waterfall at one end and skimmer at the other. I would like to install an under gravel suction grid covering the entire pond bottom that connects to an external bead / UV filter with a 4,000 gpm pump. I was going to use six 25 foot long 2 inch pipes spaced about 1 foot apart and manifolded together at both ends. I was going to have a 6 inch deep bed of 3/4 -1 inch round gravel. Are the pipes of sufficient size and spacing or do you recommend a different configuration? Is it alright to connect the filter/pump to the grid at the skimmer end or should or do I need to connect in the middle of the long dimension of the grid for a more balanced suction? Is 6 inches of gravel too deep? I realize that this only leaves 18 inches of water but the 18 inch maximum depth is being dictated by local building codes. I will have a pretty heavy fish load. Would it be better to use a bigger pump? My preference is to substantially overdo the filtration.


    Thank you for your help.
    Kirk

    1. Kirk,

      I am sorry I haven’t gotten back to you sooner. Five twenty four ft pipes will work just fine. Space so you have 6 inches from the side wall for the first one and then 12 inches on centers for the rest of them. I would think that everything should work just fine. You might want to go with a little bit larger pump depending on what external bead filter you are using. Some bead filters will cause some pretty high head pressures.

      Mike

  16. Hello,
    I am looking to build an under gravel suction grid in my pond. I live in southern New Mexico and there is sand that blows around. I will avoid sand in my yard but I fear sand will be blown in either way. I am concerned that the pipes will clog with sand in the grid. Is there a way to avoid this or a way to design it so clearing the sand will be easier than digging up the whole system?

    Thank you in advance,
    Tony

    1. Hi Mike, thank you for the two articles. I want to add a grid to the deep area of my pond. It’s about 8′ by 14′ and 4.5′ deep. I want to do this with an air lift system, you mention that th grid can easily be run off of an airlift pump that’s built correctly. But what would a correctly built airlift be for this system? I have a 100 watt air pump running five air stones now. Is it a matter of dropping an air stone into the outlet of the main pipe to move the water? I would probably divide the grid into two and use two air stones to move the grid. So my question is how is the airlift setup, what size air stones or what else is required?

  17. Hi Tony,

    I would not think that the sand would clog up the pipes as the pump would probably suck it in as the sand is much smaller than the holes. What I would see as possible problems would be that the sand could clog up the area between the rocks and give the system a problem with self adjusting flow. I think the big thing would be how much sand we are talking about and the size of the grains. I sorry I can’t give you a better answer but I have never seen this type of problem.

    Mike

  18. Mike thank you for this article and the previous article. I am in the process of installing a koi pond in place of an old above ground swimming pool. The dimensions are 16×40 and I plan on using the undergravel system of your design. I currently have the pump from the swimming pool. Would that work as a pump? Also I would like to do a pressurized under gravel system for return thru a bog. Would this work as a setup or do I just need to get rid of the big idea?

  19. Hi John.
    First let me address the swimming pool pump first. Will it work for the system and the answer is probably. Swimming pool pumps are designed to build pressure and not volume. So the question is how much volume does this pump move. Your pond is a large size pond and you are going to want to move a lot of water. I would want to move at least 9,000 gph. The second thing about a pool pump is they use a lot of electricity so a good pond pump might pay for itself in a fairly short time in electrical savings.
    Yes you can use a pressurized bog. You just have to be careful not to have any of the holes in piping pointing at all toward the bottom liner as they will make holes in the liner over time.
    Mike

    1. Mike thanks so much for the information. What pump would you suggest? I have looked for airlift pumps but I haven’t quite found one for sale. Is there any model of a pump that you suggest?

      1. Hi JohnKb,

        Air lifts are not a pump that is sold as an air lift. An air lift is the use of air to move water. Done correctly it can’t be beat for moving water. So as far as my knowledge all air lift systems are home made. They are very easily made. As far as water pumps I prefer Performance Pro but there are some other very good manufacturers out there.

        Mike

    1. I am building an upflow bog filter. I am in zone 5. How deep should the gravel bed be? Also, I am using two inch pipes with slits 1″ apart. Thank you!

    2. Lyle

      Yes

      Daniel

      On an upflow bog I like to go 3 to 4 ft deep. How wide of slits in the 2 inch pipe? Do you have a way to clean this bog filter?

      Mike

  20. Hi Lyle,
    The holes normally go 1 ft apart for each set of holes. If with that spacing enough holes can’t be installed then they can go closer. Each set of holes consists of three holes. One hole on the bottom of the pipe and one hole 60 degrees on each side of the bottom hole. How to determine where 60 degrees is measure how many inches it is around the the outside of the pipe and divide that by 6. If you have a 1.5 inch pipe it is about 6 inches around and you would drill one hole on the bottom and a hole about one inch on each side of the hole.

    Mike

  21. My husband and I created a 3300 gallon koi pond using the under gravel filter method. We used 2″ PVC pipes, a 5000 gph pump, Emperor 150 watt UV light and a biological filter. The only change we made to the plan was we placed 3-4 inch rocks between the pvc pipes and covered it with 1-2 inch smooth stones instead of pea gravel. We determined that this would allow better water flow between the larger rocks. We’ve had the pond operational for 1 1/2 years and it is crystal clear and all API test readings are perfect. We have healthy, active Koi and are delighted with the results. I did a lot of research into Koi pond designs and wanted something easy to maintain, but also very natural looking. Recently, we had to perform some underwater light adjustments and had to enter the pond. We cleared away the rocks in several places and there was no sediment between the rocks! If you hate the look of black liner, don’t be afraid to use an under gravel filter system. I would only advise you get help determining the head pressure for your filter, otherwise, you may have trouble getting enough suction into the pvc pipes.

  22. Hi Teri,

    Yes you can use the larger rock then the 3/4 to 1 inch rock. The larger the rock the larger the debris that the rock will allow thru. With the 3/4 to 1 inch rock the debris is broken down about as small as it can be broken down by bacteria. You raise a good point when you say determine the head pressure of the filter to make sure you are pulling enough water thru the gravel. Some filter especially some bead filters require a higher pressure pump to force water thru them. The multiport valve alone can require 5 psi at 95 gpm. that is a little over 11 ft of head pressure which is not much for a swimming pool pump but a lot for a pond pump. It is very common for most bead filters to require 15 to 16 ft of head pressure at higher flow rates of water. At the low flow rates the head pressure drop off.

    Mike

  23. Hi Mike
    Thank you for this article. I’m constructing a concrete 9 ft diameter pond with 1000 gallons. Besides a box skimmer filter (with two 1000 gph water pumps inside) I’m planning an undergravel airlift system. What is the recommended PVC tube diameter for the fishbone structure and the optimum air pump to run this structure?
    Regards
    Carlos

  24. Hi Mike…
    Im in the process of constructing a 2000 litre pond here in New Zealand with goldfish. I’m considering the use of a gravel filter and an in-pond skimmer (DIY version). To save electrical issues and installing more than one pump, could the in-pond skimmer connect directly to the main line of the gravel filter and suck from the surface as well as the bottom? Or does physics suggest I’ll need to attach the skimmer onto it own pump?

    In saying that… What would you recommend for a pump (size) – to suck 1700 – 2000 litres of water an hour (via the skimmer and gravel filter system to a small bog filter above the pond?

    Pond size approximately 1.8m x 1.8m x 500mm

    Thank you

    Hamish

  25. Hi Hamish,

    I would recommend that you run a line from the skimmer out to the pump and another line from the under gravel filter to the pump. I would then put valves on each line before the pump so that you can control where and how much water comes from where. I would recommend a pump that can move 2000 liters from the bottom filter and as much as you need from the skimmer.

    Mike

  26. Thanks Mike. Your reply is very much appreciated. Seems the setup I’m envisaging may seem a bit of an overkill for such a small volume of water in the scheme of things but I’m wanting as hassle free as possible.

    One other question if I may…. With being such a small volume of water really… Would you still recommend that I use and install a full size 2 inch main line for the bottom suction filter with 1/2 inch side pipes or could I get away with a modified smaller diameter system?

    I suppose now all I’m doing is guessing and playing with pure physics now, – and will just need to experiment with the system before I put in the gravel and fish!

    Knowing I need to move or pump the volume of my pond through once an hour is one thing. Taking into account waterflow through the undergravel system, up through into the bog filter above the pond and then back over the waterfall is another.

    I suppose all that is needed is a bucket and a stopwatch – then; to measure the flow rate back over the waterfall over a minute x 60: will do the same thing?

  27. Hi Hamish,

    2000 liters is about 500 gallons. So having to move at least 500 gph you can get by with a 1″ diameter pipe for the main line. But it would not bother me if volume of the pond went the under gravel filter more than once per hour. So it would not bother me if you had greater flow than 2000 liters per hour. The pond will actually work better if the flow is higher than that. So it would not bother me if the flow was up to 1000 to 1500 gallons per hour which would require a 1.5 inch main line.

    Mike

  28. Hello Mike.

    I had a pond installed this spring and we had river rock, maybe 1-3″ size in the bottom. The fish kept uncovering a portion of the piping near the pump, and I was concerned that the system was short circuiting through the open pipe.

    I added some pea gravel into the pond, hoping to increase the bio surface area, as well as hoping to prevent short-circuiting.

    I’ve had some fish die, and all tested parameters in the pond seem ok (nitrite, phosphate, ammonia, pH). I got some feedback that this type of system can harbor toxic bacteria in the ‘soil’ (gravel) in the bottom.

    two questions: do you think pea gravel is too small? And, what is your opinion about the general design, because the system is never cleaned, that it will harbor badness in the bottom?

    Thanks

    1. Hi Paul,

      If your system was installed as I outlined in the two articles you will not have any problems with bad bacteria building up in the bottom. Yes you can use some pea gravel in the bottom. I would not think that you would have any problem with the bottom clogging up because of the pea gravel but you might. If this was built correctly then I would look else where for what is causing the fish to die. My own pond has been running for the last 28 years and works perfectly and nothing has been done with the stone in the pond in over twenty years.

      Mike

    1. Melton,

      You can push water through this filter but you have to be aware that the filter will not suck up debris operating like this but put it in the water column.

      Mike

  29. Hi Mike, great article and great replies.
    Let me give you my current project info living in Pennsylvania.
    I am attaching to my home deck a new build wooden frame above ground pond using a 45 mil liner inside and pond dimensions 8′ wide x 10′ long by 24″ deep and was going to go bare liner. But after reading this great article I am considering adding an under gravel filter topped by 2″ deep of 3/4″ size river rock for the bio aspect instead of adding an external bio filter. I am going to use my Aqua Forte Sieve compact 2 filter fed by a 2300 gallon per hour pump. I could attach to the under gravel filter with a 1.5″ flex hose. I also have just purchased a 16 liter per minute air pump that I am going to connect to a Oase Oxytex 1000 under water bio filter for oxygenation and an Aquascape Iongen System g2 for algae control.

    To build the under gravel filter, what size main line and what size side fingers tubing should I use considering I am only going to have 2″ deep of top gravel on it? Do you think this offers effective enough bio filtration for goldfish to grow out to their full potential? (I don’t plan on Koi due to the water depth of only 24″)
    Also, what do you do about over wintering as I would need to shut down the water pump and only have an small heater and air pump to keep a hole in the ice for air diffusion. Is it okay to shut off the under gravel filter during the winter?
    Thanks. Dan C.

  30. Hi Dan,

    You are looking at is just under 1200 gallons of water in your pond. I would use a 2″ main line and 1.5″ fingers off of the main. As far as winterizing the piping I would construct the piping with a way of trapping air in the piping. Blow air back thru the piping until there are air bubbles coming out the under gravel filter and then trap the air in the piping and it will be just fine. You will have to drill a small vent hole in each of the fingers to let the trapped air in the fingers out so the piping doesn’t want to float up. Don’t worry about there being water in the fingers over the winter as it will be just fine the only piping that you need to get the water out of is the piping going to the under gravel filter.
    All the pump and filtering equipment is good. I won’t comment on the ion gen.

    Mike

  31. Hey! My name is Andrea. I go to a college and a friend and I just started an organization to help clean up the pond by our campus.
    One thing I noticed is that there is oil within the pond and dirt and debris. The pond is used as a dumping site for when it rains in our town however we want to figure out how it can still be that but also have a filtering system that will keep the pond clean and healthy for the life in and around the pond.
    Would this filtering system work? Or would there be another that’s more preferable? I’m not an expert so could someone also explain the effects or differences one has compared to another and how long it would take to put one in? Akon with the estimated cost and how much we would need to fundraise? Also, I know that this isn’t about the water but what else should be done to the pond besides a filtering system to keep it clean?

  32. Hi Andrea,

    Based on what you said that this pond is used as a dumping site for the town I assume that the storm water drains of the town flow into this pond as a holding pond. If this is the case then there is no telling what is in this pond. The first thing I would want to know is what kind of life is in this pond or is the pond dead? Does this pond flow into any other bodies of water. Has the pond water ever been tested as to what is in the water and how much. If there is life in the pond than the pond is not in bad shape. If the water flows into any other bodies of water then this body of water would fall under the EPA and there may be federal money available but there would also be restrictions as to what can be done.
    Since you said you are in college there may be professors there can help advise you on what can and should be done. If this is not owned by the college then the first thing is to find out who owns it.

    Mike

    1. So I asked and it is apparently a retention pond. It does not flow into any other bodies of water. There is fish in the pond however some of the fish is dead. I don’t believe that the water has been tested but I can ask.
      I will have to figure out who owns it for sure, but I do know the pond is apart of the community.

  33. Hi Mike,

    I read your articles with great interest and am convinced this is the way to go for me, as I have no real space to make a bog or settling pond.
    I’ve got two ponds adjacent each other, already stocked with koi and comets, both ponds 2.4m x 1.2m x 35 – 50 cm deep, have just installed one with an undergravel filter with an H shaped structure 2.2m in length and with two full length pipes joined in the H section with 45cm length of 25mm pvc pipe, 3 sets of 1/2′ holes every 8 inches , and connected to my 3000lt /hr submersible., just next to the H section. All this Covered in 25mm – 35mm river gravel, but have quite a few large water lilies in the gravel already, and some smaller ones in pots.
    It kind of works, but on some days the water’s really stirred up, while other days it’s as clear as can be. Do I need to pull out the lilies altogether?

    How can I improve on this system, as I am about to install into the second pond, but think I may need to Tweak the pipe sizes or something?

    Any advice would be most appreciated.

    BTW I live on the Southern Queensland coast in Oz, and it’s pretty warm here over summer…water temps hit 29 degrees C unless I have a pond full of water lettuce, which defeats the purpose of having koi…I can’t see them!

    Please help with some suggestions!

    Many thanks in advance

  34. Hi Lindi,

    The first thing I want to say is I hope that the fires there are not close to you and that this disaster ends very soon.

    In rough numbers your ponds are about 8 ft x 4 ft x 20 inches deep. You have set of pipes that are about 7 ft long that are joined together in a H pattern with 1 inch pipe. You have a pump that is drawing about 800 gph connected to the system. The holes in the pipes are a set of three 1/2″ holes that are 8 inches apart. These pipes are covered with 1 to 1.5 inch gravel. I can see a few things that are going to be problem. First the lilies will be a problem with there roots plugging the holes and clumping the gravel so that water won’t flow thru it. Second is the holes are so large and close together that four of them are about the size of the pipe. I assume that the input to the pump is connected directly to the piping and will allow debris to get to the pump and cause problems. I would guess that your pond is about 1000 to 1200 liters in volume.
    I would recommend that the holes be smaller with only 2 holes every 12″. I would go with 1/4 to 3/8″ holes. Then I also would do something with the lilies either put them in pots or get rid of them.

    Mike

  35. HI Mike,

    Thanks for your quick response, and recognition of the destruction of our beautiful wild areas not to mention loss of human lives, homes and famring and grazing properties….I’ve not been directly affected by the fires thankfully, other then difficulty with my lungs for some weeks due to the smoke,but our country is a mess right now, and tragic to see the devastation and habitat loss for so much of our endangered wildlife.

    Yes the pump is connected directly to the H piping system as you’ve described.I’ve been pondering (pardon the pun) the water liles and think I’ll have to move them into big tubs.

    Would you advise adding extra gravel layer to filter the debris further before it gets to the pipes in pond 1.
    AND would you you stick with the 1″ pipe in Pond 2 with less holes?

    Pond 2 is only 30 – 35cm deep, so don’t want to lose too much depth with heavy gravel cover for the koi in there, and would you stick with the H shape – will this work OK?

    Thanks again for your help

  36. Hi Lindi,

    The problem with the pump connected directly to the system is if anything gets into the system it is sucked directly into the pump which can cause damage to the pump. So if I had to do the plumbing so that the pump suction is directly connected to the grid I would probably put something like a p-trap right before the pump. I would probably make a 2″ trap with the u being down to have a better chance of any debris settling out. I would probably changing the 1″ pipe to 1.5 inch pipe so that a larger pump could be used now or in the future. The smaller a pond is the more often a pond must turn over.

    Mike

    1. Hi Mike,

      Thanks for your thoughts on this…what other way is there to connect the pump, if not directly to the grid system? Not sure I understand the concept, but could I plumb in a sealed/ water tight bucket between grid system and pump? Is that what you mean.
      Have had a LOT of rain since your last comment, with another 10 days of solid rain predicted and ponds are all overflowing! so might go back and re-read your initial article, to make sure I understand your plan a bit better before launching into building the filter for pond number 2, and the koi have just bred in there, so don’t want to rattle their cage too much right now.

      Thanks again for your input.

    2. Mike.
      I have a question about the underlayment on the pond floor will affect the performance of the grid holes sucking water? I use underlayment under and over the liner for protection from the large rock I use inside the pond. Didn’t know if there is a chance of the underlayment causing any issues. I assume the water weight will hold the underlayment tight to to floor and wouldn’t have to worry about in blocking any suction holes.
      Thanks for any thoughts.
      Tony

      1. Hi Anthony,

        The weight of the water isn’t going to do anything to hold underlayment in place. The wt of the gravel will. I don’t know how the underlayment is going to effect the movement of water and debris. I would probably say that it would \not have any effect but it might.

        Mike

  37. This design has been used in our designs since the 60s
    It stands the test of time and it is little to no maintenance. I installed this style in a million and half gallons of water running 3 -5hp paco pumps doing 400 gallons a min with 6” of 4/5 kiln dried granite on top. 3 years old. 184 large koi. And we never touch it. I will say I don’t believe it is best for growing show koi but for most hobbyists it works well. Any pond size commercial or residential I have installed a style system like this hundreds of times and as long as you turn over the water through it sufficiently it’s perfect, glad to see others having good success with this design. I am happy to help anyone here in CA and AZ if anyone needs it. Cheers

    Wes Glenn
    Lake Management inc

  38. Hi Mike,

    I really enjoyed both these articles, I studied them and the question and answers before building my under gravel suction grid.
    I have a pool that I converted to a pond a couple years ago, but have had a constant struggle with water clarity, light pea soup. So when I put my grid together a couple weeks ago, I didn’t empty the pond, so I did alot of it by braille.
    I used 2in pvc, put 3 holes (bottom and 60 degrees up each side) 3/8 drilled every 6 inches. Each pipe is about 10in apart. I put it in and so far have put in enough gravel to be even with the pipes and am planning on doing two more inches depth over the pipes. Ended up using 1 1/2 natural rock which is mainly smooth, due to the fact that the 3/4 smooth had about 1/3 of the rock being small enough to clog the holes. I hooked it up to a 5800 gallon per hour pump, and my pond is about 15,000 gallons but I’m going to let it drop at least a couple feet or more to decrease that.

    The actual question… I dove down today and unburried a couple different pipes enough to feel what the suction is like.. The closest to the pump had a little bit of suction, but around the half way point in the pool, I wasn’t feeling any suction. So should there be a lot of suction? Before doing the next step of adding more rock, I’m wondering if I should be corking up a lot of the holes, maybe half, so the 3 holes are every foot? Won’t be fun but I’ll do it if that’s what needs to be done.

    The pond did clear up pretty good in about a week, not crystal yet, but way better than its ever been.

    Thanks!

  39. Hi Tory,

    How many holes do you have and what is the total surface area of all the holes. I like to have the the total surface area to be five to ten times the surface area of the main suction pipe used in the system. The one thing I see that could be a problem is the holes are 6 inches apart and not 1 ft. That would mean twice the number of holes.

    Mike

  40. Thanks for getting back to me! I just added it all up and the surface area of all of the holes is way more than 10 times the 2 inch pipe… I came up with a contingency, I have a bunch of 3/8 nylon plugs on there way to me, I’m gunna be plugging a lot of holes.

    Have you ever felt the suction from the holes in a suction grid? For when I have many of my holes plugged, should there be a great amount of suction from all the holes, like if you put something light in front of a hole a few inches away should you see it suck it towards and into it? Or 6 inches away?

    Thanks, I’ve found it is very difficult to get many specifics on suction grids, your articles and Q&A gave more than I was able to find anywhere else. Thanks so much for keeping up on your Q & A!

  41. I’m 2/3 of the way through plugging holes in my suction grid. But the one thing I still don’t know is…Have you ever felt the suction from the holes in a suction grid? For when I have many of my holes plugged, should there be a great amount of suction from all the holes, like if you put something light in front of a hole a few inches away should you see it suck it towards and into it? Or 6 inches away?

    Even after plugging about 2/3rds of the holes, I still feel very little suction on the holes closest to the pump, and even half way between I’m not feeling any suction from the holes. I want to make sure I’m doing this right so I don’t put my fish in danger.

    Also I upgraded to a 2 hp whisperflo which puts out close to 10,000 ghp and have let my water drop about 1.5ft so far getting me just about to all the water cycling through hourly.

    Thanks!

  42. Hi Tory,

    You not going to feel a suction until you get down to all the holes adding up to less then the surface area of the 2 inch pipe. The reason I use 5 to 10 times is because of the rock blocking some the holes either wholly or partly. With 2 inch pipe the 10,000 gph pump is still only going to move about 5000 gph. What size holes do you have?

    Mike

  43. Hi Mike,
    Thanks, my holes are 3/8 in, and currently the section I have running is probably about 10 times the 2in pipe in holes. So then even if I dig down and feel some of the holes and there is no suction… Do you need to feel suction or is it OK without suction from the holes?
    The pump has a 2in input and output, how would it not pull it’s maximum gph from 2in pipe?

    Appreciate your help

  44. Hi Tory,
    Okay with that hole size and pipe size then the total max number of holes should 284 holes.

    The pump has 2 inch in and out fittings and yes if you were pulling water right at the pump and the output was measured right at the pump it would be very close to 10,000 gph. What you run into is a thing called head pressure. What this is is the amount of power to lift so much water a given height. Yes I know you are not lifting the water very high so why would the volume drop off much. The problem is the friction of water flowing thru the pipe. The greater the volume of water thru a pipe the greater the pressure that is needed to move a given amount of water. Granted your pump is a 2 hp pump but it still has some limits. Your pump needs at least 30 ft of head pressure and will produce 8800 gph. The charts for 2″ pipe don’t go that high a volume but I would guess that 30 ft of head would be less then a 100 ft of pipe. The next thing you are going to say is that you have less then a 100 ft of pipe. The problem is each fitting has more friction than straight pipe. As an example a standard 90 degree elbow is the same as 6 ft of pipe. Change that to a sch 40 90 degree elbow and you are at 13 ft of pipe.

    Mike

    1. Hi Mike

      Thanks for the great articles and your time replying to everyone.

      Can I ask your thoughts in using your design in comparison to using 100mm perforated land drain pipe instead? Any pro’s and con’s etc?

      There seem to be a lot of natural swim ponds currently being built using a bubble pump and 100mm perforated land drain pipe in a regeneration zone with aquatic plants above. This was the design I was about to start with but now I’m even more confused which is the best method. Could I still do it as planned with a regeneration zone with stones and plants above but using your pipe layout method?

      If I’m going to build my 20 metre swim pond, I want to do it right so would really value your opinion and thoughts of the 2 different pipes?

      Thanks.
      Andy

  45. Hi Mike

    I love your system and would love to use it in my natural pool that i am about to begin construction on in regional NSW Australia.

    The base grid will go on the bottom of the deepest end of my pool that will be 91000lt in total volume 7x10x2m and 7x3x1m swimming area. I was thinking 2.5″ centre pipe with branches 3.5m wide every 300mm out of 2″ pipe. with 3 x 10mm holes drilled every 300mm as you suggest bottom and 60deg each side. I would cover with gravel 3/4-1″ and riverstones on top for smoothness.

    I want it to then pump into a bog, base of bog 50mm above waterlevel, that will rise through 300mm gravel and cascade into the pool via a waterfall. I thought of using 3 x 10m long 3″ pipe with slits cut across the top every 150mm with an angle grinder then covered in 3/4-1″ gravel for growing plants in.

    Will this be suitable do you think? i thought i would need a pump that would move 91000lt/hour or more, would you agree?

    I appreciate your time.

    Belinda

    1. Hi Belinda,

      I am not sure what is happening but I have tried answering your question twice before and it has not shown up here.

      Since some of the people including me do not normally use the metric system I am going to convert some of your post to US measurements. First the pond that you are building is about 24000 gallons of water. The piping being constructed with normal measurements. Then the water would pump into a bog filter with the bottom of the bog about 2 inches above the pond and then rise up 12 inches thru gravel and flow back into the pond. The water would flow thru 3 pipes that are 3″diameter and about 31 ft long with slits cut across the top of the pipe every 6 inches. The pump used to run the system would move about 24,000 gallons per hour.
      I hope that I have converted everything about correctly. The first thing that I want to look at is how the piping going to handle the flow rates that the pump you are talking about. Since you don’t say what say what else if anything this pump will be running I am going to base my following comments on this pump running only the under gravel and bog only. First let’s look at the suction side of the pump. 2.5 inch center pipe can handle about 7500 gallons per hour. So this pipe would have to be increased to at least 4″ pipe. The branches off of it might have to be increased in diameter depending on how many branches there are.
      Now lets talk about talk about the bog. There you are using 3″ pipes and 3 of them. With the right piping going to the piping in the bog and the right size manifold connecting these pipes it will handle the water. The only thing that I would change is to put the slots on the bottom of the pipes and not the top.
      You have done a nice job laying this out and it sounds like it will work nicely.

      Mike

  46. hi mike
    im reza from indonesia…
    so your pump move water from the bottom of the pipe, to next filtration?
    or should i just use the undergravel pipe with airlift, and my other filters running separately?
    thx mike

  47. Hi Antareza

    If I understand your question correctly you are asking should you take the water from the under gravel filter and run it thru another filter or should both run separately. The real question is are both filters design to the same thing as far as filtering something from the water. If they are both doing mostly the same filtering then I would run the water one to another as there will not be much for the second one to do. By separating them the water is getting filtered twice as fast which can be very good.

    Mike

      1. Hi Antareza,

        I am not sure that I understand your question but I assume that you are asking do I put anything inside the pipes to grow bacteria. No the pipes are empty other then water. Of course the inside walls will grow bacterai.

        Mike

  48. thx mike.
    curently building indoor pond inside my room that goes through my childs room. lol.
    50cm x 500cm. the depth around 60cm.
    with bottom drain, drainage, standar small 4 chamber filtration, and still working on under gravel. but got consfuse on the process.
    thx for your answer mike…

    reza

  49. Hi Antareza,

    I am not sure how a bottom drain with an undergravel filter will work. The filter will block any debris from getting to the bottom drain. If the bottom drain is going to be used to drain water from the pond that it will do.

    Mike

  50. Hi Reza

    In order to use the under gravel filter to drain water from the bottom you need to put a waste output on the pump so that the pump will suck up water from the bottom and then the pump pumps it to waste. This is easily done.

    Mike

  51. Hi

    Can I ask your thoughts in using your design in comparison to using 100mm perforated land drain pipe instead? Any pro’s and con’s etc?

    There seem to be a lot of natural swim ponds currently being built using a bubble pump and 100mm perforated land drain pipe in a regeneration zone with aquatic plants above. This was the design I was about to start with but now I’m even more confused which is the best method. Could I still do it as planned with a regeneration zone with stones and plants above but using your pipe layout method?

  52. Hi Andy,

    Can you use 100 mm perforated pipe with the under gravel filter. Normally I believe that perforated pipe has 5/8 inch holes that are drilled closer together. So in a very short distance the surface area of all the holes would be greater then the 10 times the cross section surface area of the pipe. 100 mm pipe can easily handle flow rates of about 20,000 gph. So if you had a flow rate of about 5000 gph with this pipe the water would not be moving but in about 1/4 to 1/3 the length of the pipe. Then you had in the fact that there are going to have plants above is going to be a problem. If you are sucking water thru the pipe the roots of these plants are going to want to plug the holes. Normally a system like this would be used to pump water thru the gravel and plants. I hope I have given you clue as to when and when not to use this type of system.

    Mike

  53. Hi Mike,
    Thank you so much for your clear instructions. You’re about the only one on the internet providing us, DIY pond builders, with this precious information.
    I have a concrete agricultural basin of 30 by 42 feet, 6,5 feet deep ( total of approx 62 000 gallons) . I would instal 4 individual undergravel suction grids, each run by air lift pumps, that provide a flow 4000 gph (total water movement of 16 000 gallons /hour).
    Each of the 4 grids grids would be made as follows: central line 4″diameter, 30 feet long, with branchs each side in 2 inch pipe, 5 feet long, with 3/8 inch diameter holes (3 holes) every feet in the branches. Smooth :3/4 to 1,5 inch gravel around and on top. Il will make sure to make son airlift pump skimmers also, ans floating plants on surface.
    Do you think my design will be adequate?
    Thanks so much for your feedback
    Axelle

  54. Axelle,

    The system you are describing could work nicely. I kind of question if 4000 gph per branch is enough water movement. It should work well but you will be using just a small part of the grid at any given time.

    Mike

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