Nitrogen cycle
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The diagram above shows how the Nitrogen cycle works in an aquarium. Water is drawn in through the filter and through various media which has been colonized by different types of bacteria that absorb the waste products and convert them into other products. Waste and Ammonia (NH4) are produced when a fish digests its food and this is converted by Nitrosomonus bacteria into Nitrite (NO3) . A different type of bacteria, nitrobacter, then converts the Nitrite (NO3) into less harmful Nitrates (NO2) . Ammonia and Nitrate are both lethal to fish above fairly low levels however Nitrate is not as harmful so long as it is not allowed to build up. Plants feed on Nitrates so they can absorb some and the rest can be kept in check by changing the water for fresh water which is nitrate free. One sign that you may have higher than neccasary Nitrates is Algae on the tank glass and over rocks and accesories in your tank.
