How to Cycle a Fish Tank

The nitrogen cycle in brief

Fish waste and leftover food produce ammonia, which is toxic. One group of bacteria turns ammonia into nitrite, also toxic. Another turns nitrite into nitrate, which is far less harmful and is removed with water changes. These bacteria live mostly in the filter media and gravel, not in the water, and a new tank doesn't have them yet.

Fishless cycling, step by step

1. Set up the tank, filter and heater. Add dechlorinator to tap water.
2. Add a source of ammonia: pure household ammonia or bottled aquarium ammonia, dosed to about 2 ppm.
3. Test ammonia and nitrite every couple of days with a liquid test kit.
4. When ammonia starts falling and nitrite appears (usually week 1 to 2), keep dosing ammonia back to 2 ppm.
5. Eventually nitrite also drops. The tank is cycled when it can turn 2 ppm ammonia into zero ammonia and zero nitrite within 24 hours.
6. Do a large water change to lower nitrate before adding fish.

How long it takes

Usually 4 to 8 weeks. Keeping the water around 78 to 82°F (25 to 28°C) during cycling speeds it up. Bottled bacteria products or a handful of used filter media from a healthy tank can cut it to a week or two.

After cycling

Add fish gradually, a few at a time, so the bacteria can keep up. Never rinse filter media under tap water; chlorine kills the bacteria. Rinse it in tank water you've removed during a water change. To plan how many fish your tank can take, use the aquarium stocking calculator.