The aquarium nitrogen cycle

How the nitrogen cycle works

Fish and leftover food release ammonia. Bacteria in the filter turn it into nitrite, and other bacteria turn nitrite into nitrate. Nitrate is far less harmful; plants and water changes remove it. Ammonia is more dangerous the higher the pH and temperature.

Stages of cycling

Fishless cycle step by step

1. Run the filter and heater. 25–28 °C (77–82 °F) speeds up bacterial growth.
2. Add an ammonia source: pure ammonia with no additives to about 2 mg/l (ppm), or a pinch of fish food.
3. Test ammonia and nitrite every 1–2 days. When ammonia drops, add another dose.
4. The cycle is done when, a day after a dose of about 2 mg/l, ammonia and nitrite both read 0. Nitrate showing up alone doesn't prove it.
5. Do a large water change to lower nitrate and add fish gradually, a few at a time.

Media from a mature filter of another tank shortens the cycle by bringing ready bacteria.

Common mistakes

• Adding fish to a brand-new tank straight away.
• Rinsing filter media under the tap: chlorine kills the bacteria. Rinse it in tank water.
• Turning the filter off for many hours: without flow and oxygen the bacteria start to die.
• Adding all the fish at once: the bacteria can't keep up with the new ammonia.

How Aquarium Monitor helps

The cycle assistant in the app records ammonia doses and test results, tells you which stage the tank is at, what to do next and when the cycle is really finished. The ammonia → nitrite → nitrate diagram shows where the chain stalls, and you can read test strips from a photo.