Cycling grows the bacteria in a new filter that turn fish waste into nitrate, which is safer. Run the tank without fish, test the water, and add fish only once ammonia and nitrite stay at zero.
How long it takes
The sources don’t agree, and RSPCA says there’s no exact rule. Plan for the longest and let the tests decide.
| Source | How long | In its words |
|---|---|---|
| RSPCA | At least two weeks | “There is no exact rule for how long cycling will take but it will be at least two weeks.” |
| RSPCA Australia | Usually 4 to 6 weeks | Fully cycled once nitrates are being produced and ammonia and nitrite are zero, which “usually takes 4 to 6 weeks”. |
| UF/IFAS Extension | Six to eight weeks | “A new biofilter requires six to eight weeks to build up sufficient bacteria to effectively reduce ammonia and nitrite levels.” |
| OATA | Several weeks | The fishless method adds “a calculated dose of ammonia to your aquarium over several weeks”. |
Steps
- Set up and wait. Fill the tank and start the filter. OATA advises leaving a new aquarium 24 to 48 hours to settle.
- Cycle without fish. RSPCA strongly advises new owners to use fishless cycling, running the filter for a while before any fish go in. OATA’s fishless method adds a calculated dose of ammonia over several weeks. It says cycling with a few hardy fish should only be done with great care and a shop’s guidance.
- Test the water. Test for ammonia and nitrite every other day (RSPCA), or every two to three days for the first six weeks (RSPCA Australia).
- Watch the numbers. Ammonia and nitrite rise and then fall, while nitrate usually keeps rising (OATA).
- Know when it’s done. The filter is mature once ammonia and nitrite have dropped to zero and stay at zero (OATA). RSPCA adds fish only after a spike in both, followed by a return to zero.
- Add fish slowly. Don’t add a lot of fish at once: RSPCA Australia says gradual introduction lets the filter bacteria adapt.
Why it matters
RSPCA Australia says the number one cause of fish deaths in new aquariums is a filter without enough bacteria, letting ammonia build up. It says ammonia is toxic to fish at 0.1 ppm, and UF/IFAS Extension says nitrite is toxic at levels as low as 0.10 mg per liter.
UF/IFAS adds that the un-ionized form of ammonia is about 100 times more toxic than the ionized form, and that its share rises as pH or temperature rises.
Once it’s cycled
Keep testing and change some water every week: RSPCA says 30% weekly, and OATA up to 25% every week. When you clean the filter, never rinse its media under a tap, which washes away and kills the bacteria (OATA).