The Nitrogen Cycle is the process that is responsible in keeping your fish happy and healthy. Fish produce ammonia. Unfortunately ammonia is highly toxic and can lead to the death of your fish. Low concentrations of ammonia will begin to burn the gills of fish and cut off their oxygen supply. Bacteria found in oxygen rich areas then begin to eat ammonia, creating a by-product known as nitrite. Nitrite is also toxic to fish and can affect the health of the fish and eventually lead to its death. Bacteria found in oxygen deprived areas then begin to eat nitrite, converting it to a less harmful nitrate. Nitrate is removed through absorption from plants and through water changes.