Before adding fish, a new aquarium needs a colony of beneficial bacteria that turns toxic ammonia into nitrite and then into less harmful nitrate. Fishless cycling adds an ammonia source, then you test until ammonia and nitrite both read zero within 24 hours of dosing and nitrate is present. It often takes four to eight weeks. Bottled bacteria can speed it up.
New Tank Syndrome kills more fish than any disease. Cycling the tank first prevents it.
The cycle
| Stage | Process |
|---|---|
| Ammonia | Produced by fish waste and decaying food; toxic |
| Nitrite | Bacteria convert ammonia to nitrite; toxic |
| Nitrate | Other bacteria convert nitrite to nitrate; removed by water changes and plants |
Fishless cycling
- Set up the tank with filter, heater and substrate; add dechlorinated water.
- Add an ammonia source as directed by the product or guide you follow.
- Test ammonia, nitrite and nitrate every few days.
- Keep the ammonia source going as nitrite rises and then falls.
- Finished when ammonia and nitrite read zero within 24 hours of a dose, and nitrate is present.
- Do a large water change, then add fish gradually.
Speeding it up
- Bottled nitrifying bacteria
- Filter media or sponge from a healthy established tank
- Warm water within the bacteria’s range and good oxygenation
Test kits
Liquid kits are more accurate than strips. Test ammonia, nitrite, nitrate and pH as a minimum. Reef tanks need more; see reef parameters.
Choose a filter sized for the tank with our filter guide. Browse water and testing.
Common questions
How long does it take to cycle an aquarium?
Often four to eight weeks, sometimes faster with bottled bacteria or media from an established tank.
How do I know my tank is cycled?
It can process a dose of ammonia to zero ammonia and zero nitrite within 24 hours, with nitrate rising.
Can I add fish while cycling?
Fish-in cycling exposes fish to ammonia and nitrite. Fishless cycling is kinder and more predictable.
What test kit do I need for cycling?
A liquid kit that tests ammonia, nitrite, nitrate and pH.