How to cycle an aquarium: the nitrogen cycle, step by step
AquaLog guide · Published
In short: Cycling grows the bacteria that turn fish waste (ammonia) into nitrite and then into much less harmful nitrate. A fishless cycle usually takes 4–8 weeks. It is finished when ammonia and nitrite both read 0 ppm within 24 hours of adding ammonia, and nitrate is rising.
Why does a new tank need cycling?
Fish release ammonia through their gills and in their waste, and uneaten food breaks down into more of it. Ammonia burns gills even at low levels. In a mature aquarium, bacteria living in the filter and on every surface convert it to nitrite (also toxic) and then to nitrate, which is far less harmful and is removed by water changes and plants. Research on aquarium filters found the second step is done mainly by bacteria of the genus Nitrospira, not the Nitrobacter named on many older bottled-bacteria labels.
A brand-new tank has almost none of these bacteria. Add a full load of fish on day one and ammonia builds up faster than anything can remove it. Fishkeepers call the result new tank syndrome. Cycling means growing the bacteria first.
What do you need?
- A liquid test kit for ammonia, nitrite, nitrate and pH. Strips are quicker but less precise.
- An ammonia source: pure ammonia sold for fishless cycling, or plain household ammonia with no scent or surfactants. Fish food also works but is harder to control.
- Dechlorinator, because chlorine and chloramine in tap water kill the bacteria you are trying to grow.
- The filter running day and night, and the heater set around 26–28 °C (79–82 °F). Nitrifying bacteria grow faster in warm water.
How do you do a fishless cycle?
- Set the tank up completely, fill it with dechlorinated water, and start the filter and heater.
- Add ammonia until the test reads about 2 ppm. Test again the next day to confirm.
- Test ammonia and nitrite every two or three days. Ammonia starts to fall as nitrite appears, usually within one to three weeks.
- Whenever ammonia drops below 1 ppm, top it back up to about 2 ppm. The bacteria need a steady supply or they starve.
- Nitrite usually climbs, peaks and then falls. This second stage is often the slowest. If nitrite gets very high and progress stalls, a partial water change can help.
- The cycle is complete when, 24 hours after dosing 2 ppm ammonia, both ammonia and nitrite read 0 ppm and nitrate is clearly above zero.
- Do a large water change to bring nitrate down, then add your first few fish.
For how much ammonia to add from the bottle you have, a day-by-day testing log and what to do when a cycle stalls, see the fishless cycling guide.
How long does cycling take?
Usually four to eight weeks, sometimes longer. It goes faster with filter media or decor from an established, healthy aquarium, which carries live bacteria. Bottled bacteria products can help, though results vary. Warm water helps, and so does a pH that stays above about 6.5: nitrifying bacteria slow down sharply in very acidic water.
Can you cycle a tank with fish in it?
You can, but it is harder on the fish. Start with only a few hardy fish such as zebra danios or platies, feed lightly, test every day, and change water whenever ammonia or nitrite reaches about 0.25–0.5 ppm. Expect it to take longer than a fishless cycle.
How do you keep the cycle going?
- Rinse filter media in old tank water, never under the tap.
- Never replace all of the filter media at once.
- Do not leave the filter switched off for hours; the bacteria need oxygenated water.
- Some medications, especially antibiotics, can damage the bacteria. Test more often after treating.
Next: learn what your test results mean in aquarium water parameters explained, and settle into a routine with the maintenance checklist. You can log every reading in AquaLog and see the cycle progress on a chart.