Cycling grows the bacteria that turn toxic ammonia into nitrite, then into nitrate. The tank is cycled when it clears 2 ppm of ammonia to 0 ppm ammonia and 0 ppm nitrite within 24 hours and nitrate has risen, usually after 4 to 6 weeks. Cycle without fish if you can.
Here’s how to cycle a fish tank: run the filter and heater, add an ammonia source, and test until ammonia and nitrite both read 0 and nitrate appears. That usually takes 4 to 6 weeks, and it’s the fishless method, the safer one. If fish are already in the tank, you’re doing a fish-in cycle, which needs daily testing and water changes to protect them.
New tank? Do this straight after setting it up. Already seeing an ammonia reading with fish in the water? Jump to the fish-in section, and read how to reduce ammonia in a fish tank for the emergency steps.
1. What Cycling a Fish Tank Actually Means
The nitrogen cycle is the process where bacteria convert ammonia from fish waste into nitrite, and then nitrite into nitrate. Cycling a tank simply means growing enough of those bacteria, mostly on the filter media and surfaces, before fish arrive. Ammonia and nitrite are both toxic to fish, while nitrate is far milder and is removed by water changes.
| Stage | What converts it | Toxic to fish? |
|---|---|---|
| Ammonia from waste and food | Nitrifying bacteria, first group (Nitrosomonas) | Yes, damages gills |
| Nitrite | Nitrifying bacteria, second group (Nitrospira) | Yes, interferes with oxygen in the blood |
| Nitrate | Removed by water changes (and plants) | Mild, harmful only at high levels |
Skip it and the fish become the ammonia’s only audience. RSPCA Australia says to test every two to three days for the first six weeks in a new tank, then every one to two weeks. Its targets are ammonia under 0.1 ppm and nitrite under 0.2 ppm.
2. How to Cycle a Fish Tank Without Fish (Fishless Method)
This is the method to use whenever there are no fish yet. It puts nothing at risk, and it gives you a clear pass mark.
- Run the tank. Fill it with treated water, switch on the filter and heater, and hold about 26°C, because warmth speeds the bacteria up. Keep pH above 6, since the bacteria slow right down below that.
- Add an ammonia source. The usual target is 2 to 3 ppm. Use pure household ammonia, checking the label lists no detergent, perfume or colour, or ammonium chloride. Fish food works too but is slower and messier. Add a little, test, and top up until you reach the target.
- Test every 2 to 3 days. Track ammonia, nitrite and nitrate with a liquid kit. Ammonia will fall, nitrite will appear, then nitrite will fall as nitrate rises.
- Keep feeding the bacteria. When ammonia drops to 0, add enough to bring it back to 2 ppm. Don’t go higher, because too much ammonia can kill the bacteria you’re growing.
- Run the pass test. The tank is cycled when it turns 2 ppm of ammonia into 0 ppm ammonia and 0 ppm nitrite within 24 hours, and nitrate has risen.
- Dilute the nitrate. Do a couple of large water changes (up to 50 per cent each) with treated water at the same temperature. Then add fish, a few at a time.
Bacteria starters can help a new tank, and so can filter media from a healthy established tank. Neither replaces the pass test. The beneficial bacteria roundup compares the options.
Planted or betta tank? The steps shift a little.
How to Cycle a Fish Tank With Fish (Fish-In Method)
Sometimes the fish are already home. Fish-in cycling exposes them to ammonia and nitrite, which is why some fishkeeping societies call it poor practice. If you’re committed to it, your job is to keep both readings as low as you can while the bacteria catch up.
- Keep numbers small. Only the fish you already have, lightly fed, once a day at most. Adding more just adds more ammonia.
- Test daily. Check ammonia and nitrite with a liquid kit.
- Change water whenever readings climb. Use RSPCA Australia’s targets of ammonia under 0.1 ppm and nitrite under 0.2 ppm as your trigger. Then change 25 to 50 per cent with treated water, which is University of Florida IFAS guidance for small systems.
- Check your tap water. SA Water says chloraminated water often carries free ammonia that many conditioners don’t fully remove, so a big change can add ammonia. Test a bucket of treated tap water first.
- Hold off on new fish until ammonia and nitrite both stay at 0 for several days, with nitrate present.
Expect fish-in cycling to take longer, and check your state’s rules before buying any fish, because keeping some species is restricted in parts of Australia and the rules differ by state and change over time.
| Fishless | Fish-in | |
|---|---|---|
| Risk to fish | None | Ammonia and nitrite exposure |
| Effort | Dose and test every 2 to 3 days | Test daily and change water often |
| Clear pass mark | Yes, the 24-hour test | Several days at 0 ammonia and 0 nitrite |
| Best when | The tank is new | Fish are already in and you have no alternative |
How Long Does It Take to Cycle a Fish Tank?
Most tanks take 4 to 6 weeks, and some take longer. UF/IFAS says a biofilter needs 6 to 8 weeks to become effective. Warmth around 26°C helps, as does a healthy source of seeded media. Cold water, a pH under 6, or too much ammonia slows it down. The full timeline has its own guide: how long it takes to cycle a fish tank.
If you’re stuck, test pH and hardness first. Nitrification uses up the water’s buffering, so a falling pH can stall the process. The pH & Hardness hub explains what to test, and the Tank Cycling hub has more cycling guides.
Common Mistakes
- Adding fish at the first sign of nitrite. The cycle is only half done.
- Dosing too much ammonia. Above 2 to 3 ppm can stall the bacteria.
- Using household ammonia with detergent or perfume. Check the label.
- Rinsing filter media under the tap. Chlorine and chloramine harm the bacteria, so use old tank water.
- Turning the heater off or letting the tank run cold and cutting the bacteria’s speed.
When to Call It
If ammonia or nitrite still isn’t moving after 8 weeks, check the filter is running, the pH is above 6 and the temperature is about 26°C. Take a water sample to a local aquarium shop for a second opinion if it still won’t budge.
Related Guides
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Frequently Asked Questions
How do I know if my fish tank is cycled?
It’s cycled when 2 ppm of ammonia falls to 0 ppm, with 0 ppm nitrite, within 24 hours, and nitrate has risen. Test again after a water change to confirm the readings hold.
How long does it take to cycle a fish tank?
Usually 4 to 6 weeks, and up to 8 in some tanks. Warm water, seeded media and a pH above 6 all help. Test every 2 to 3 days rather than counting days on a calendar.
Can I cycle a fish tank quickly?
You can shorten it, but not skip it. Seeded filter media from a healthy tank and a bacteria starter can help, and warmth around 26°C speeds things up. Prove the result with the 24-hour ammonia test.
What happens if you do not cycle a fish tank?
Ammonia and nitrite build up, and both are toxic to fish. That can damage gills and kill fish, which is why new tanks need testing every 2 to 3 days for the first six weeks.
What do I use as ammonia for a fishless cycle?
Pure ammonia or ammonium chloride, dosed to about 2 to 3 ppm. Check that household ammonia lists no detergent, perfume or colour. Fish food also works, but it’s slower and less predictable.
