| Biological filtration can incorporate one or more | | | | Nitrate is considered less harmful to fish, and |
| media on which the bacteria will grow. Whichever | | | | keeping the level well below 100 mg/l and |
| media you choose, their function will be to colonize | | | | preferably less than 50mg/l is recommended for |
| the aerobic or oxygen loving bacteria that oxidize | | | | koi. Although they can cope with higher levels than |
| the ammonia to produce nitrite. The flow rate | | | | this for short periods, avoid it is possible. Because |
| through the filter must be slow enough to allow | | | | nitrate is a plant food, external vegetable filters |
| the bacteria to do their work. | | | | can help to keep it under control. |
| Nitrite is also highly toxic and should not be | | | | They can also double up as an effective backdrop |
| allowed to increase in level as it will damage blood | | | | to a pond or form a stand alone water feature. |
| cells of the koi, which can lead to internal organ | | | | Single cell and filamentous algae will also thrive on |
| damage. Even in extremely small concentrations, | | | | nitrate. Pond plants and algae will be eaten by the |
| nitrite is still highly toxic to fish and must be | | | | koi and the cycle begins again. The nitrogen cycle |
| closely monitored. | | | | can best be summarized as follows. |
| You may register an increase in nitrite level when | | | | The total ammonia starts at zero with no fish, no |
| a new filter is going through the maturation | | | | plant life, but rises with time when fish are added |
| process or when a mature filter has to cope with | | | | and as feeding is increased. Then, the bacteria |
| an additional fish load. The next stage of the | | | | start to multiply and as their food supply |
| nitrogen cycle is the colonization of the bacteria | | | | increases, they oxidize the ammonia and the build |
| that further oxidezi the nitrite to nitrate. | | | | up of nitrite begins. |