The nitrogen cycle is essential for the well being of all fish. It converts ammonia from fish waste into nitrite and then nitrite into less harmfull nitrates. Beneficial bacteria (called Nitrosomonas) convert ammonia, which is extremely harmfull to fish, into nitrite.Nitrite is also harmfull to fish but this is converted to nitrate by a different bacteria called nitrobacter. Nitrate is the end product and this is less harmfull to fish. Nitrate can be removed by a process called denitrification but the conditions required for this to occur are difficult to replicate. For a healthy tank our nitro bacterias require oxygen and this is normally provided in our aquariums by filters which pump oxygen rich water through the media containing the beneficial bacteria. Denitrifying on the other hand requires a much slower rate of flow. The best way of dealing with nitrates is to change some of the water in the tank frequently so that the nitrates are diluted. Plants will use nitrates to grow so the addition of plants will further reduce nitrates. When you start up a tank it essential that you cycle it, allowing the slow reproducing bacteria to build up before you introduce your fish. Introducing too many fish before the tank is cycled, will lead to an explosion of ammonia with perhaps not enough bacteria to efficiently run the nitrogen cycle. Rising ammonia levels will without doubt kill your fish within a couple of days. Nitrate build up above 30 parts per million will eventually affect your fish inhibiting growth and will impair your fishes immune system. Your fish will be stressed and will suffer ill health. NOTE these beneficial bacteria can be wiped out if you clean your filter media in tapwater. ALWAYS clean your filter media in tank water.