Filtration is primarily the most important part of Koi keeping.

There are several types of suitable filters, these are Biological, Botanical and Mechanical.

The basic rule is the more biomass of fish you keep, the larger or more efficient your filter needs to be.

A good article by the Koi Doc was: I view Koi as extremely efficient sewage making machines. We throw in high quality, expensive food and the Koi convert it into high quality water polluting sewage. If we are to maintain good water quality, we need to remove or neutralise the sewage as fast as the fish produce it, otherwise there will be a steady increase in unwanted pollutants.

A biological filter will take the wastes from the fish and process it naturally to produce clean water. This is done through micro organisms (bacteria) that develop in the Biological Filter.

The process is called the nitrogen cycle. The bacteria that you are building a chamber to house, will turn deadly Ammonia into deadly Nitrites and then into less harmful Nitrates on a continual basis. The end product (Nitrates) will need to be removed with a minimum of 10% weekly water changes.

A Biological Koi filter system should have a surface area of approximately 25% of the pond that it is going to service.

All filters should have a funnel shaped base with controllable drainage.

Although the Bacteria in a filter is only active down to a limited depth, it is advisable if practical to make the filter chambers as deep as possible. This will increase the volume of water in the system and therefore assisting to dilute any pollution.

It is advisable to install what I call a primary filter, prior to your main filter chambers. This can be in the form of a Vortex or a chamber with suspended media, large and easy to lift out material such as fish netting, brushes, ect.

This primary filter can be cleaned on a regular basis by opening a tap for a vortex, or stopping the system and dunking the media, then opening a waste tap in the case of a chamber filter.

A primary or mechanical filter can collect up to 75% of all the solids from your pond, this will largely reduce the work load on your filters.

It is most important to have an air supply fitted to a filter system as bacteria can not survive without it. If you have multi filter chambers without air stones, most of the oxygen will have been used up in the first chamber.

Never wash your filter with tap water or chlorinated water. Use your pond water or dechlorinated water and remember it does not have to be spotless.

All of your water changes (10% of your pond water weekly) should be done by draining your filters.

Pumping from a pond to the filters has a tendency to break up the solids and it is harder for the primary filter to trap the waste in a finer form.

A primary filter will be most affective if you are gravity feeding the filters from your pond and pumping back from the last chamber or designated pump chamber. Remember the water may not be able to run through your filters fast enough to keep up with your pump in the pump chamber, therefore you should install a bypass pipe from your vortex to the pump chamber. This must have a tap on it which will allow you to control the flow of water through your filters and if need be, supply enough water to your pump chamber.

Today there are many modern filters that are very good and take up less space. The one I have described here is a basic biological jap mat filter.

It is most important to install a good filtration system. Over time you will become a water keeper first and a koi keeper second.

The internet has a world of information and there are many books and advice available at your local Koi Club.

Bill Passey


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