Biological Filtration and the Nitrification Cycle

So how does a bio-filter work?

A biological filter is quite simply the heart of a koi pond. It is not essential in small fish ponds, but the more fish you stock, the larger they get and the more they eat, so the need for a bio-filter becomes greater. The pond gets to a point where it needs a "sewage farm". It's purpose is to convert the waste matter produced by the koi from harmful ammonia into less toxic waste.

There are 2 types of bacterial species that colonise the biological filter media.
Nitrosomonas sp. bacteria which oxidize ammonia to nitrite,
and
Nitrobacter bacteria convert nitrite to nitrate.

Ammonia

Ammonia (NH3) is produced by fish (and particularly koi because they are fat greedy chaps!), as part of their normal metabolic function and is excreted from the gills. The amount of ammonia produced is directly related to the amount of food they eat. Approximately 3-4% of normal 30-40% protein level koi food will be excreted as ammonia, i.e. for every 100grams of food 3-4grams (3000-4000mg) of ammonia is produced.

Koi exposed to unacceptable levels of ammonia risk damage to gills, eyes, fins and skin which can result in them being susceptible to secondary bacterial infection.

Nitrite

Ammonia is oxidized by the Nitrosomonas sp. bacteria in the filter to produce nitrite (NO2).

Whilst it is not considered as dangerous as ammonia it can still do serious damage to your fish. High levels of nitrite are likely to stress your koi leaving them susceptible to secondary infection. As with ammonia, target levels should be that nitrite is undetectable.

Before the fish pond filter can efficiently remove ammonia and nitrite from the fish pond water, it must first become fully colonized with nitrifying bacteria. This can take some time and is a process known as fish pond filter "maturation". Each time a fish is put in the fish pond it will add to the total amount of ammonia being produced. The ammonia level in the fish pond will therefore increase slightly. Because there is more ammonia for the bacteria to utilize, they start to multiply until there are enough to use all of the ammonia being produced inside the fish pond. The ammonia level in your fish pond will then fall back to zero.

Nitrate

As the ammonia level falls, the amount of nitrite produced by the bacteria in the fish pond filter will start to increase. Therefore, the level of nitrite in the fish pond will rise. The increasing nitrite level means that the bacteria that break it down can start to multiply in the fish pond filter until, as with the ammonia, there are enough to use up all the nitrite that is being produced. The nitrite level within the fish pond can then fall to zero. As this occurs, the nitrate level increases.

Conversion of nitrite to nitrate (NO3) is the final stage of the nitrification process. There is debate as to the possible problems that elevated levels of nitrate may cause. Indeed some koi keepers have high Nitrate and it causes no problem at all. High nitrate may also attribute to green water (phytoplankton) and blanketweed growth however the two do not always go hand in hand. The green water problem can get worst when you clean the biofilter and make water change outs, due to the reduction in bacteria.

The bacteria also produces a certain phytoplankton-killing enzyme. As algae starts to grow in the bio-filter, or on the walls of the pond, the bacteria loves to feed on this algae, and as it does so it releases the enzyme into the water.

Green water is a pain for many reasons. Ultra Violet Clarifier lights will kill single cell phytoplankton algae that cause green water, and when dead they clump together and can be removed by the filter. However there is sometimes a concern expressed that passing water through the UVC also kills beneficial bacteria. Note that a UVC does not get rid of blanketweed.

The Skippy site teaches us that we should try to achieve "balance" in the pond - don't fight mother nature.

By use of the bio-filter and other larger plant forms you starve the water of Nitrate, so that the algae has no food, and is therefore unable to grow, while at the same time the bacteria create the enzyme which kills the phytoplankton. Its a double-edged sword in this battle!