Before adding fish, a new aquarium needs a colony of beneficial bacteria that turns ammonia into nitrite and then nitrate. During fishless cycling, add an ammonia source and test until ammonia and nitrite read zero and nitrate is present, following a consistent cycling protocol. It takes several weeks in many systems, and temperature and water conditions affect the timing.
The filter needs an established community of nitrifying bacteria to process fish waste. Until that biological filter develops, ammonia and nitrite can rise to harmful levels.
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.
- Follow your cycling protocol until ammonia and nitrite test at zero and nitrate is present; cycle timing varies with water conditions and the ammonia load.
- Do a large water change, then add fish gradually.
Speeding it up
- 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.
Related reading
Common questions
How long does it take to cycle an aquarium?
Often several weeks; temperature, water conditions and system load affect timing. Established filter media may shorten startup, but verify progress with water tests.
How do I know my tank is cycled?
After a measured ammonia input, ammonia and nitrite should return to zero with nitrate present. Follow a consistent fishless-cycling protocol and stock gradually.
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.