How long does a fish tank take to cycle?

Usually four to eight weeks. A new tank has none of the bacteria that turn fish waste into less harmful nitrate. Cycling grows them before any fish arrive.

What happens during the cycle

1. Ammonia rises. Fish waste and rotting food release ammonia, which harms fish even at low levels.
2. Nitrite rises as ammonia falls. The first bacteria turn ammonia into nitrite, which is just as harmful.
3. Nitrate appears as nitrite falls to zero. A second group of bacteria turns nitrite into nitrate, which fish tolerate in moderate amounts and water changes remove.

How to cycle a tank without fish

1. Set up the tank with its filter and heater running, and treat tap water with a dechlorinator.
2. Add a source of ammonia: a pinch of fish food every day or two, or bottled pure ammonia dosed to about 2 ppm.
3. Test every two or three days with a liquid kit that reads ammonia, nitrite and nitrate. Write the numbers down.
4. Keep adding the ammonia source. Expect ammonia to peak first, then nitrite, then both to fall.
5. The tank is cycled when ammonia and nitrite both read 0 within a day of adding the ammonia source, and nitrate reads above 0.
6. Change about half the water to bring nitrate down, then add fish a few at a time, a week or two apart.

Readings that mean the tank is cycled

What makes it faster

• Filter media or gravel from an established, healthy tank carries the bacteria with it.
• Keeping the water warm, around 25 to 28 °C (77 to 82 °F), helps the bacteria grow.
• Rinse filter media in old tank water, never under the tap: chlorine kills the bacteria.

If fish are already in an uncycled tank

Test daily. Whenever ammonia or nitrite reads above 0, change part of the water with dechlorinated water at the same temperature, and feed lightly. Add no more fish until both read 0 for a week.

After cycling

Add fish gradually so the bacteria can keep up, and keep testing weekly for the first month. The planner answers "not ready to stock" until you mark the tank as cycled.