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Koi Pond Filters

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Clear water in your Koi pond does not necessarily mean clean water, it may contain colorless impurities, such as ammonia and nitrite, that are harmful and can kill koi. Koi excrete urine and produce faeces in the Koi pond, and ammonia is excreted through the gill membranes. It is the job of the pond filter to remove waste from the Koi pond which in the wild would be diluted by the large volume of water or washed away by moving water.

Most pond filter media involved in maintaining a Koi pond have a mechanical function. Settlement chambers allow gravity to drag the solid waste out of the Koi pond 's water by slowing the water flow. Such chambers usually come first in a filter. A vortex unit provides greater settlement, the water in the Koi pond moves in a circular movement allowing solids to gather in the centre where they can be removed. In addition to baffle plates within the pond filter which slow the incoming water, brushes or matting can be used to strain the water of the Koi pond.

Biological filtration of the Koi pond relies on specific bacteria to break down toxic waste products to less harmful substances. The first stage is the breakdown of ammonia to nitrite in the Koi pond by nitrifying bacteria, most important of which is Nitrosomonas. The second stage is the conversion of nitrite to nitrate by Nitrobacter. Both of these groups of bacteria are aerobic (need oxygen to live), sediment building up in the Koi pond 's pond filter will deplete the oxygen levels so it is important to keep sediment in the pond filter to a minimum by having a settlement chamber first and by cleaning the filter out occasionally (but not using tap water as the chlorine will kill the bacteria).

A variety of different Koi pond filter media are available to put in the filter, materials such as gravel, matting, hair rollers, foam, and Canterbury spar are all suitable as they provide lots of surfaces for the bacteria to live on.

A biological Koi pond filter will take weeks or months to mature cultures of nitrifying bacteria are widely available and will speed up the process. The process of Koi pond chemical filtration involved activated carbon removing ammonia and other organic waste products by adsorption, this means that the waste substances become linked to the surface of the carbon. When the surface is 'full up' it has to be replaced.

If a large Koi pond biological filter is present chemical filtration should not be needed, but it is good to use while the biological filter is maturing or isn't big enough for the pond. Some Koi pond keepers use a sand filter as a final stage to 'polish' the water. The Koi pond's water is passed under high pressure through sand and comes out very clear, bacterial activity also takes place in the sand filter. Sand filters are expensive though, and you can't make one yourself because of the high pressure involved.

Biological filtration turns Koi pond's ammonia into nitrate which is harmless to fish (unless at extremely high levels) but the disadvantage of this is that algae love nitrate and you get an algal bloom. There are two types of algae problems, green water and blanket weed. Green water is caused by microscopic algae in the water, it is not harmful to koi, actually it is beneficial, the koi eat the algae and it enhances their colour, but you can't see them! Also in summer the algae use oxygen and leave the Koi pond fish gasping. There are various ways to get rid of the algae: a vegetable filter, plants will use the nitrate so it is not available for the algae; an ultra violet (UV) filter kills the algae as it passes through; algicide chemicals can be used but the problem will just recur; magnets placed on the filter pipe will disrupt algae cells internally, killing them or preventing them reproducing.

Blanket weed is filamentous algae and forms long green strands, it is not really a problem, it uses up nitrate and stops green water occurring, but it is unsightly. Vegetable filtration, algicides and magnets will all work on blanket weed, but UV filtration will not as the algae has to pass through the filter to be killed and blanket weed is attached to the pond walls.

Koi pond filters need to be cleaned occasionally to remove sediment, take this into account when building one. Add a bottom drain to each filter chamber so that sediment can be let out, it makes cleaning much easier. Another thing that makes cleaning easier is to put filter medium in net bags, not just pour it in all at once, as it can then be more easily removed, one bag at a time.

One last important thing, never ever put tap water in a mature filter, it will kill all the bacteria and you will have to let it mature all over again.

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