OK, so this article is to describe the Nitrogen Cycle, but in longer words, some my kids will not understand.

Fish most living creatures eat and breath. That is where the problem starts! The food they don’t eat, the leaf from the live plant, and the food that the fish eats and then comes back out starts to rot and one of the byproducts is Ammonia, however surprisingly this is not where most of the ammonia comes from, that comes from the fish breathing!

This Ammonia, can be very bad for the fish, even in small quantities it can and will kill them. However depending on the temperature and the pH of the water defines how toxic it will be for the fish.