Biological filters are not the only place in the koi pond and filter system where biological filtration takes place. Nitrifying bugs (nitrosomonas and nitrobacter) are prolific in nature, on land, everywhere in the aquatic environment, even in the air but it is only how they behave under water that is of interest to koi-keepers.
In a koi pond, they colonise any wet surface including the floor, walls and even inside pipe-work. In fact, they prefer the inside of pipe-work to open surfaces because they are photophobic, which means that they don’t like light. Light doesn’t harm them, but they prefer dark places and will multiply better without light. These bugs can swim because they have little whip-like tails that they can rotate and propel themselves through water. They can also take part in the nitrogen cycle whilst swimming but are more efficient when they can form a biofilm on solid surfaces. They clearly can’t think or plan where they are going but they will propel themselves around seeking to avoid the light until they chance upon a dark place and settle onto a solid surface where they form their colonies. Ensuring our bio-filters are dark is the first step in encouraging bacteria to settle there and grow into mature colonies.
It is often thought that these bugs use either ammonia, (in the case of nitrosomonas) or nitrite (in the case of nitrobacter) as a food but, in fact, what really happens is that they break apart these chemicals and this releases a form of chemical energy which allows them to capture carbon from carbonates in the water which is what they really need in order to grow and multiply. For a bio-filter to be able to support a mature bacterial colony there must be a plentiful supply of carbonates. Ensuring that the KH is sufficiently high is essential. A bacterial colony cannot mature without carbonates and even a mature one will not remove ammonia from the water without a supply of these.
As well as a form of carbonate in the water it is vital that the water flowing through the bio-filter is also well oxygenated, in processing ammonia through nitrite to nitrate, bio-filter bugs use over four times as much oxygen and over seven times as much carbonate as the ammonia they process.
If all these conditions are in place, the bio-filter will quickly mature from naturally occurring bugs without external aids such as seeding with a filter booster or starter.
Once a filter has matured, there is a common problem experienced each spring in unheated koi ponds. Filter bacteria don’t die during winter but become less active. As water temperatures fall to around 5°C they protect themselves by going into a dormant stage and shutting down their internal biochemistry in a way that allows it to restart when the water warms again. Once shut down, they are effectively in a state of suspended animation and cannot restart activity until the temperature rises significantly.
Koi begin feeding and excreting ammonia before this temperature is reached resulting in a springtime ammonia spike. There is little koi-keepers can do during these couple of weeks except keep feeding to a minimum so as to keep the ammonia spike below about 0.5 mg/L. If this level isn’t exceed, the koi will come to no harm.