How to cycle a new fish tank

Cycling grows the good bacteria that turn fish waste into something safe. Skip it and new fish often get sick or die in the first few weeks. It takes patience, but it's simple.

What "cycling" means

Fish waste and leftover food break down into ammonia, which is toxic to fish even in small amounts. In a healthy tank, bacteria living in the filter turn ammonia into nitrite (also toxic), and a second group of bacteria turns nitrite into nitrate, which is much less harmful and gets removed with water changes.

A brand-new tank has none of these bacteria yet. "Cycling" is the process of growing them before you add fish. This is also called the nitrogen cycle.

What you need

• Your tank, set up and running with the filter and heater on
• Water conditioner (dechlorinator) to remove chlorine from tap water
• A liquid test kit that measures ammonia, nitrite and nitrate. Test strips are less accurate.
• An ammonia source: either pure household ammonia (no soap or scent) or a pinch of fish food every day
• Optional: bottled beneficial bacteria, which can speed things up

Fishless cycling, step by step

This is the kindest method, because no fish are exposed to ammonia while the bacteria grow.

1. Fill and treat the tank. Add water conditioner, then turn on the filter and heater. Set the heater to about 78–82°F; bacteria grow faster in warm water.
2. Add ammonia. Dose pure ammonia a few drops at a time until your test reads about 2 ppm. If you're using fish food instead, add a pinch each day.
3. Test every 2–3 days. Write down your ammonia, nitrite and nitrate readings. After a week or two, ammonia starts dropping and nitrite appears.
4. Keep feeding the bacteria. Whenever ammonia falls close to zero, add more to bring it back up to about 2 ppm.
5. Watch for nitrite to fall. This is usually the slowest part. Nitrate will start to climb, which is a good sign.
6. Do the 24-hour test. Dose ammonia to 2 ppm. If ammonia and nitrite both read 0 the next day, and nitrate is present, your tank is cycled.
7. Do a big water change (about 50–75%) to lower the nitrate, then add your first fish within a day or two so the bacteria don't starve.

How long does cycling take?

Usually 4 to 8 weeks. Warm water, bottled bacteria, or a handful of used filter media from a friend's healthy tank can shorten it. Cold water and low pH can slow it down.

Can I cycle with fish in the tank?

Yes, but it's harder on the fish and on you. Start with just a few hardy fish, feed lightly, and test every day. Whenever ammonia or nitrite reads above about 0.25–0.5 ppm, do a water change to bring it down. Expect to change water often for several weeks.

Common cycling problems

• Nitrite is stuck high for weeks. It's normal for this stage to be slow. A partial water change can help if nitrite climbs very high.
• Cloudy white water. This is usually a harmless bacteria bloom that clears on its own in a few days. See cloudy aquarium water.
• Nothing is happening. Check that the filter is running, the water is warm, and you're using conditioner, since chlorine kills the bacteria.

Next: Everything you need to start a fish tank · Best fish for beginners