A well-balanced tank contains organisms that metabolize the waste products of other inhabitants. Nitrogen waste is metabolized in aquaria by a type of bacteria known as nitrifiers (genus Nitrosomonas). Nitrifying bacteria metabolize ammonia into nitrite. Nitrite is also highly toxic to fish in high concentrations. Another type of bacteria, genus Nitrospira, on?converts nitrite into less?toxic nitrate. (Nitrobacter bacteria were previously believed to fill this role, and appear in "jump start" kits. While biologically they could theoretically fill the same niche as Nitrospira, it has recently been found that Nitrobacter are not present in detectable levels in established aquaria, while Nitrospira are plentiful.) This process is known in the aquarium hobby as the nitrogen cycle.
I do be thinking so FL. To be honest after so many years in the hobby I really did need somebody to remind me of this most important fact in fishkeeping. The very first thing you should ever learn, such a shame it wasn't brought to my attention until now.