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

Question

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

In: 2

Answer 1

Fish poop out ammonia, which can very quickly build up to toxic levels in an aquarium. When a tank has been cycled it hosts a colony of nitrogen-fixing bacteria in its filter and substrate. The bacteria oxidize the ammonia into nitrate and then nitrite, which is harmless. “Cycling the aquarium” means establishing the bacterial colony on a new tank, typically by transferring filter media from an older aquarium.