The nitrogen cycle

Fish make ammonia. In a new tank nothing breaks it down yet, so it builds up and burns their gills. Cycling grows the bacteria that turn it into something harmless.

How to cycle

1. Set up the filter, heater and dechlorinated water.
2. Add an ammonia source: pure ammonia to 2 ppm, or a pinch of food each day.
3. Test every 2–3 days. Ammonia rises and falls, then nitrite does the same.
4. When ammonia and nitrite read 0 within 24 hours of dosing and nitrate shows, the tank is cycled.
5. Change most of the water, then add a few fish at a time.

Bottled bacteria and filter media from a healthy tank speed it up. Never clean the filter under tap water: chlorine kills the bacteria.

Stages at a glance

Feed the bacteria. Add an ammonia source: a pinch of fish food daily or pure ammonia to 2 ppm. Test every 2–3 days.

Ammonia is being eaten. The first bacteria are growing. Keep ammonia around 1–2 ppm and wait for nitrite to appear.

Nitrite is up. The second bacteria are growing. Nitrite will rise, then fall. Don’t change water unless it passes 5 ppm.

Almost there. Ammonia and nitrite read 0 and nitrate is showing. Dose to 2 ppm; if both are 0 again in 24 hours, you’re cycled.

Your tank is cycled. Two clean tests a day apart. Do a big water change, then add fish a few at a time.