Koi

Koi produce waste from the breakdown and ‘burning’ of foods to release energy. Most of this waste is in the form of ammonia, a substance that is incredibly toxic to fish. In nature, this ammonia would be diluted and washed away by huge volumes of water, but in ponds where we keep fish in relatively small volumes of water, the ammonia levels accumulate.

Fortunately, some types of bacteria break down ammonia to get energy – they add oxygen and form a substance called nitrate. Furthermore, a different sort of bacteria turn this nitrite into nitrate by adding more oxygen. Both bacterium live everywhere but can be given a home and ideal conditions in a filter.

Nitrate can be used by plants for food. Ultimately then, you start with food (fed to Koi) and finish with food (fed to plants), so there is recycling going on. This process is called the nitrogen cycle, since nitrogen is the element common to ammonia, nitrite and nitrate.

All waste is a potential source of ammonia, and the waste you can see (fish waste, uneaten food, and so on) is just the tip of the iceberg. Most of the ammonia released by Koi exits from the gills and you can’t see it.

All three substances (ammonia, nitrite and nitrate) can be measured with widely available test kits. Follow the manufacturer's guidelines because the toxicity of ammonia varies with the pH of the water (in acid water, ammonia is less toxic). In any case, generally you don’t want any ammonia at all (0.0mgl), nitrite should be less than 0.1mgl and nitrate should be less than 20mgl. With all three, always aim for zero. The odd brief peak in levels probably won’t kill fish immediately, but it’s vital to find out what’s causing the blip and sort it out.

High levels mean there’s too much waste and not enough filtration. Causes may be too much food, too many fish, too little water going through the filter, too small a filter, or that the filter’s blocked, dirty or isn’t working well (it could be too cold, too low in oxygen, or damaged by chemical treatments, for example).

In the long term, at least one of these areas needs to be looked at if you find levels of ammonia, nitrite and nitrate creeping up. In the short term, carry out a water change (10–20% every day) and maximise aeration in the pond because both these things will reduce a Koi’s stress.

Written by Adrian Love