Biological Filtration - The Nitrogen Cycle

The nitrogen cycle describes how organic wastes break down in the aquarium. Fish wastes naturally decompose into ammonia, which is highly toxic for marine life. Nitrosomonas bacteria process the ammonia into nitrite, which is also toxic. Nitrobacter bacteria then break down the nitrite into nitrate, which is much less harmful. This is as far as the cycle goes in most tanks, though under the right conditions, the nitrate is further broken down to free nitrogen gas.