Eli5: What does it mean to ‘cycle an aquarium’?

Question

Eli5: What does it mean to ‘cycle an aquarium’?

In: 2

Answer 1

It literally just refers to removing all of the water and replacing it with clean filtered water.

This needs to be done fairly regularly because there are a lot of toxic chemicals that can build-up in water and not be obvious until they start killing the life in the aquarium. Ammonia is a pretty big and important example, and it is always building up because all animals secrete it in their urine.