Nitrogen cycle

The nitrogen cycle is the biological process that makes an aquarium safe to hold fish and shrimp. Waste from fish, uneaten food and decaying plant matter breaks down into ammonia, which is toxic even in small amounts. Colonies of naturally occurring bacteria convert that ammonia into nitrite, which is also harmful, and then a second group of bacteria converts nitrite into nitrate, which is far less dangerous and removed mainly through water changes.

This process takes time because the bacteria have to grow and multiply on surfaces inside the tank, especially in filter media, substrate and on hardscape. A brand new aquarium has almost none of these bacteria present, which is why adding a full stock of fish on day one so often ends in a cloudy tank and dead fish within the first week or two. Testing ammonia, nitrite and nitrate with a liquid kit is the only reliable way to know where a tank stands in the cycle, since none of these compounds are visible to the eye.

New tank owners should watch for an ammonia spike in the first week, followed by a nitrite spike as the second bacteria colony catches up, and finally falling nitrite alongside rising nitrate as the cycle completes. A common mistake is cleaning filter media too aggressively or replacing it too often, which strips out the bacteria the whole system depends on and can restart the cycle from scratch.