Fish waste, uneaten food, and decaying plant matter all produce ammonia. Ammonia is poison to your fish and must be removed. When ammonia is present in an aquarium, a bacterium called Nitrosomonas will start to form. Nitrosomonas oxidizes ammonia and puts off a byproduct called nitrites. Nitrites are also poisonous to fish but less than that of ammonia. In the presence of nitrites, a bacterium called Nitrobacter will form which converts nitrites into nitrates. Nitrates are significantly less poisonous to fish and are removed from a tank by doing water changes. Plants will also feed off of nitrates so having a planted tank can slow the build-up of nitrates in an aquarium [1]
