The nitrogen cycle is the most important thing in keeping fish happy and healthy. What happens is a bacteria grows that converts the deadly ammonia into nitrite, but that is still bad, so another bacteria eats that and turns it into nitrate.
See fish poo ammonia through their skat, then like i stated the bacteria will turn it into nitrite then nitrate. But if the has not happened and there is no benificial bacteria then ammonia levels will build up and kill your fish.
Hope this helps
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