Cycling a new aquarium: the nitrogen cycle step by step

The number one mistake beginners make is filling the aquarium, waiting a couple of days and adding fish. The almost certain result: deaths from ammonia poisoning. The solution is to understand and complete the cycling process.

What the nitrogen cycle is

Fish and food scraps produce ammonia (NH₃), which is toxic. In a mature aquarium, colonies of bacteria live that transform it:

1. Ammonia → Nitrite (NO₂) — Nitrosomonas bacteria. Nitrite is still toxic.
2. Nitrite → Nitrate (NO₃) — Nitrobacter/Nitrospira bacteria. Nitrate is much less toxic and is controlled with water changes (or plants/corals).

Cycling is, quite simply, giving those bacteria time to establish themselves before having fauna that depend on them.

How to cycle (without fish, the recommended way)

1. Set up the aquarium, filter and your usual temperature.
2. Add a source of ammonia (dosed pure ammonia, or starter bacteria + some food).
3. Test every 2-3 days for ammonia, nitrite and nitrate.
4. You will see ammonia rise, then nitrite, and finally nitrate appear.

The aquarium is cycled when, within 24 h, ammonia and nitrite return to 0 and only nitrate remains. It usually takes 3-6 weeks.

Signs that it is NOT ready

• Nitrite is still detectable.
• Ammonia does not drop to 0 within a day.
• It has been less than 2-3 weeks.

Be patient. A good cycle is the difference between an aquarium that gives problems all the time and one that is stable for years. Record every test to see the curve clearly: when nitrite drops to zero, you can start introducing fauna little by little.