Koi fish pond

One of the most important decisions you’ll make when building a koi pond is the type of filter to use. Before we even start discussing types, though, let’s make one thing clear in your mind – water filtration is an essential part of keeping koi. While you may get away with keeping a few small goldfish in an unfiltered pond, that will not work with koi for several reasons.

- Koi release a lot of waste into the water, which obviously affects the water quality. In particular, high levels of ammonia and nitrites can kill your fish.
- Koi have been so inbred to achieve the colors and varieties available that their immune systems are weak and easily shocked. Excellent water quality is a must to keep them healthy.
- On a much lighter note, solid wastes in the water will cloud the water, making it difficult or impossible to see your koi in the pond.

Because of the size of koi ponds – and the high quality of water necessary to keep koi healthy – you’ll need to budget for a high quality water filtration system. The inexpensive ‘black box’ systems that are designed for simple garden ponds aren’t designed for the rigorous needs of a koi pond. They’re not meant to handle the volume of water needed for even a small koi pond. Nor do they do a good job of mechanical filtration – sifting out the solid wastes and debris that fall into the pond.

Filtration for a koi pond is a two step process: first water passes through a ‘mechanical’ filter which removes solid particles from the water, then through a biological filter where ‘friendly’ bacteria remove the ammonia and nitrites from it.

Mechanical filters function rather like a sieve. The water is drawn or pushed through a screen that retains the solid particles while allowing the water to pass through. The finer the screen/net, the better a job of filtering it will do. There are two types of mechanical filters – gravity and pump filters. Gravity filters are generally considered to be superior in result than pump filters.

Biological filters consist of a medium of some sort on which nitrite and ammonia eating bacteria grow and thrive. The media used to provide a home for the bacterial colonies are varied, but many experts recommend Japanese matting and Kaldanes K1.