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

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.

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