Why does a new fish tank need weeks before fish? The nitrogen cycle

A new tank looks ready the moment the water is clear, but it has no helper bacteria yet, and the first fish waste can poison it.

Cycling is the weeks-long process of growing bacteria that turn toxic ammonia into nitrite and then into much milder nitrate. Slide through a simulated cycle, see why colder water takes longer, and learn what a finished tank reads on a test kit.