To try to put it simply, fish waste and other decomposing organic matters like decaying plants or uneaten fish food will release ammonia into the water.
Probably one of the most important topics to learn in the aquarium hobby is the nitrogen cycle. To try to put it simply, fish waste and other decomposing organic matters like decaying plants or uneaten fish food will release ammonia into the water. Ammonia is toxic and will kill the livestock in the tank. Bacteria (known as “beneficial bacteria”) living on/in the filter media, substrate, and any other surface in the aquarium, will convert ammonia into nitrites and then eventually nitrates.