The cycle in one paragraph
Fish release ammonia through their gills and waste, and leftover food adds more. Ammonia is poisonous. Bacteria living on the surfaces of your filter media convert it to nitrite, which is also poisonous. A second group of bacteria convert nitrite to nitrate, which fish tolerate in moderate amounts and which you remove with water changes. A tank is "cycled" when it has enough of both groups to convert all the ammonia its fish produce, all the time.
A new tank has almost none of these bacteria. They arrive on their own, but they multiply slowly: building a full population takes four to eight weeks.
What you are aiming for
| Test | Cycled tank | New or struggling tank |
|---|---|---|
| Ammonia | 0 ppm | Anything above 0 |
| Nitrite | 0 ppm | Anything above 0 |
| Nitrate | 5–40 ppm, rising between water changes | 0 (cycle has not started) or very high (water changes overdue) |
Use a liquid test kit. Paper strips are quick but often miss low levels of ammonia.
How to do a fishless cycle
This is the kindest and most reliable method: you feed the bacteria with bottled ammonia, and no fish is exposed to toxins.
You need: the tank running with filter and heater, dechlorinated water, a liquid test kit, and pure ammonia sold for cycling aquariums (ammonium chloride), dosed by the bottle's instructions.
- Set the tank to 26–28 °C (79–82 °F). The bacteria multiply faster in warm water. Keep the filter running day and night.
- Dose ammonia to about 2 ppm. Test after an hour to confirm.
- Test every day or two. For the first week or two, nothing seems to happen. Then ammonia starts to fall and nitrite appears.
- Whenever ammonia falls below 1 ppm, dose back up to 2 ppm. The bacteria must be fed continuously or they die back.
- Wait out the nitrite spike. Nitrite often climbs off the top of the chart for a week or two. Then it drops, sometimes overnight, and nitrate appears.
- Test for completion. Dose 2 ppm of ammonia. If ammonia and nitrite both read zero 24 hours later, the cycle is complete.
- Change most of the water to bring nitrate under 20 ppm, set the heater to your fish's temperature, and add the first fish within a day or two. If fish are delayed, keep dosing a little ammonia.
To speed it up: move some filter media, sponge or gravel from a healthy, established tank into your filter. That transfers a working colony and can shorten the cycle to a week or two. Bottled bacteria products sometimes help and sometimes do nothing; established media is the dependable shortcut.
If the cycle stalls
- pH has dropped below about 6.5. The process produces acid, and the bacteria slow sharply in acidic water. A partial water change restores the buffer.
- Ammonia or nitrite is far too high. Above about 5 ppm, the bacteria are inhibited. Change half the water and continue with smaller doses.
- Chlorine. Untreated tap water, or media rinsed under the tap, kills the colony. Always use conditioner.
- The filter was off. The bacteria need oxygen-rich flowing water. A filter left off for some hours can lose much of its colony.
Cycling with fish already in the tank
If you already have fish in an uncycled tank, you can bring them through it with care.
- Test ammonia and nitrite every day.
- Change 30–50% of the water whenever either reads 0.25 ppm or more. Daily if need be. Use the water change calculator for the amount.
- Use a water conditioner that also binds ammonia and nitrite, dosed for the full tank volume.
- Feed lightly: once a day, a small pinch. Less food means less ammonia.
- Add no more fish until ammonia and nitrite have both read zero for a week.
Keeping the cycle healthy
The colony grows to match the fish load and no further, so:
- Add fish gradually. Doubling the fish overnight outruns the bacteria and causes a "mini-cycle". Plan the order with the stocking calculator.
- Never replace all the filter media at once. Rinse it gently in a bucket of tank water when flow drops. Replace sponges only when they fall apart, and then half at a time.
- Keep the filter running. After a power cut of more than a few hours, test for ammonia for several days.
- Be careful with medication. Some treatments, especially antibiotics, harm filter bacteria. Test during and after treatment.
Questions and answers
How long does it take to cycle a fish tank?
Usually four to eight weeks from scratch. With filter media borrowed from an established tank, it can take as little as one to two weeks.
How do I know when my tank is cycled?
Dose ammonia to 2 ppm. If both ammonia and nitrite test at zero 24 hours later, and nitrate is present, the tank is cycled.
Can I cycle a tank without ammonia?
You need some source of ammonia to feed the bacteria. Bottled ammonium chloride is the cleanest. A pinch of fish food left to decay also works, but it is slower and harder to control.
Do water changes slow the cycle?
No. The bacteria live on surfaces, not in the water. Water changes during cycling are helpful if ammonia or nitrite climb too high or the pH falls.
What should nitrate be in a cycled tank?
Above zero and ideally under 20 ppm, with 40 ppm as the usual upper limit for freshwater fish. Regular water changes keep it there.
Do live plants help?
Yes. Plants take up ammonia and nitrate directly, and arrive carrying some bacteria. A heavily planted tank cycles more gently, but still needs testing before fish are added.