They say that the key to keeping koi is water quality. Then the key to water quality is adequate and appropriate filtration.

Any filter system (and the emphasis is on system) should include the following four, essential elements:

Bottom drains – A 4,000-6,000 gallon pond might get along fine with just one bottom drain if constructed so sediment was kept moving toward it. Ponds above 6,000 gallons should have at least two bottom drains gravity feeding to two separate filtration systems. If two or more are used, they should never be connected by a “Y” but taken all the way to separate (or one very large) settling chambers in at least 4″ schedule 40 or 80 PVC pipe. Four inch drains should have an anti-vortex dome that is supported by a central pedestal and not the type supported by legs at the edge of the dome. They become traffic jams which eventually stops flow. Drains should gravity feed to the filter – if you pump to a filter you puree all the poop and stuff making filtration – spelled EXTRACTION – more difficult.

Tip: A word on gravity feeding. The basic rule is, water will always seek its own level. If you place two containers (or even more) side by side (such as a pond and a settling chamber) and run a pipe from one to the other(s) anywhere below the water line, and fill them with water, the water level will even out from one to the other. If we pump from one, the water from the other(s) will flow to compensate and that’s how a gravity flow recirculating system works. As long as the pump is running, the filter system water level will always be slightly below pond level as the pond water is always trying to catch up. How much difference depends on the flow rate of the pump. The higher the output of the pump, the lower the water drops in the filter system containers. Example: 2400 gallons per hour (GPH) will drop the level about 1 inch. Note: A new (slick) 4″ PVC pipe can carry about 3500 GPH by gravity. The flow rate will reduce as the pipe starts growing things inside.

Settling chambers – The most efficient is called a vortex (whirlpool). Water enters on a tangent about two thirds of the way down the side of the container, causing a swirling motion forcing the larger pieces of crud to move out to the sides where gravity draws them down to the bottom where the purge line enters the cone shape of the purpose built container. When we see a build-up of debris, we just pull the knife valve in the 3″ (minimum) purge line and get rid of it to waste. For most Koi ponds, this container should be a minimum of 40″ in diameter and 40″ deep. The point is to slow the water down enough for the heavy stuff to drop out and any smaller container is ineffective when pond water flow rate is at the typical 2000-2400 gallons per hour (2400 GPH is maximum for a 40″ vortex). The rule of thumb is, the larger the vortex the greater water flow we can have and still accomplish the same result.

Mechanical filtration – This is where we actually strain or extract or trap or take something out of the water. We actually want particles to cling to whatever we place in the path of the water. The choices of material are numerous but my choice is cylindrical (usually 4″ in diameter) brushes with a stainless steel core and bristles of nylon or other similar synthetic material. It’s best to buy the thick, good ones as they’ll stop more stuff and they never wear out. They come in various lengths to suit your needs and can be used in up-flow, down-flow or horizontal applications. You’ll want at least four rows, each one slightly enmeshed or overlapped with the other from side to side. And with brushes, more is better. They can be hung in place with dowels or metal (non-rusting) rods. However, they must be cleaned from time to time and because we’re not asking them to perform any biological function, a garden hose and chlorinated water is okay if flushed away from the system (chlorinated water will kill the good guy bacteria in the biological processing station).

Biological processing – Here’s where the chemicals you can’t see such as ammonia and nitrite, are eaten by “good-guy” bacteria provided by Mother Nature. Remember, every surface under water anywhere in the pond – this means streams, waterfalls, the sides of the pond, anything under water – is a place for “good-guy” bacteria to reside and work for you. But because we usually have too many fish in too small a body of water, this surface area is insufficient to do the job. So – what most of us do is provide a container of some kind of material outside the pond, on which the bacteria can colonize. What kind of material? Ask ten different people and get ten different answers. The rule of thumb is to get the most surface area for the smallest volume. I like Japanese or domestic matting or the ribbon-like media for their light weight and ease in cleaning (even though we give this container the cleanest possible water, over time crud will accumulate and we’ll have to clean it). I don’t like lava rock or any kind of gravel/aggregate because it tends to clog and channel and is tough to take out of a container and try to clean. (How do we know if our processing station is doing its job? Test the water. Inexpensive test kits for ammonia and nitrite are readily available and should be used routinely and should always show zero contamination). It’s from this processing station that we’ll pump back to the pond and create the gravity flow/recirculating function.

Tip: Ready made filter systems are available but size is critical. Be doubtful of anyone who shows you a 2′ x 2′ x 3′ box and tells you it will take care of 6000 (or whatever) gallons. This might work if you only want a couple of fish. Ask instead, how many mature, 24″ fish, being fed normally, that the filter system can support. There is no formula and little science to help us decide on size and shape so talking to experienced koi keepers is the best approach.

Tip: You can have as many fish as your filter can support but, a crowd looks like a crowd. Fifteen, 24″ fish in a 25′ x 13′ x 3′ pond looks great. Fifty, 12″ fish looks like rush hour on Times Square.

In addition to selecting a site for the pond, you need to decide where the filter system will go. It can go most anywhere -out in the woods, around the corner of the house, maybe in the garage – but it should either be concealed or suitably camouflaged so as not to intrude in the beautiful setting you’re making. The use of a surveyor’s transit will come in handy to make sure the water level in the filter system will be the same as the pond. These can be rented at equipment rental places and this is critical to the efficiency of a gravity fed recirculating system.