How to cycle a fish tank (simple step-by-step)

Quick answer

Cycling is the process of growing beneficial bacteria that convert toxic ammonia into nitrite, then into nitrate. To cycle a tank, run the filter, add an ammonia source, and test water until ammonia is 0 and nitrite is 0, while nitrate is present. Most tanks take 2 to 6 weeks.

What you need

• Filter running 24/7
• Dechlorinator (chlorine kills beneficial bacteria)
• Liquid test kit: ammonia, nitrite, nitrate
• Ammonia source: fish food or bottled ammonia made for cycling
• Patience: stability beats speed

Step 1: start the tank and stabilize temperature

• Fill the tank, add dechlorinator, and start the filter.
• If keeping tropical fish, set the heater and let the temperature stabilize.
• Run the tank for 24 hours before you begin testing.

Step 2: add an ammonia source

Pick one method:

• Fish food method: add a small pinch daily. It breaks down into ammonia.
• Bottled ammonia method: follow the label instructions for cycling.

The goal is to create measurable ammonia so bacteria can grow.

Step 3: test on a simple schedule

• Test ammonia and nitrite every 2 to 3 days.
• When you first see nitrite, that is progress.
• Later, you will see nitrate appear. That is also progress.

Step 4: know when cycling is complete

Your tank is cycled when:

• Ammonia reads 0
• Nitrite reads 0
• Nitrate is present

A practical confirmation is this: after adding a small ammonia dose, the tank processes it back to 0 ammonia and 0 nitrite within about 24 hours.

Step 5: do a partial water change before adding fish

• Do a partial water change to reduce nitrate.
• Keep the filter media wet and avoid rinsing it in tap water.
• Add fish slowly, not all at once.

Common mistakes

• Adding fish before the cycle is complete
• Cleaning filter media with tap water
• Replacing all filter media at once
• Overfeeding during the first month
• Skipping water tests and guessing

FAQ

Can I cycle a tank with fish in it?

It is possible, but it is harder and risky. A fishless cycle is safer and more controllable.

Why does my tank show ammonia but no nitrite yet?

Early on, there may not be enough bacteria to process ammonia. Keep the filter running and continue testing. It often takes a week or two before nitrite appears.

Should I add bottled bacteria?

It can help, but it does not replace testing. Always confirm with ammonia and nitrite readings before adding fish.

What nitrate level is acceptable?

Lower is better. Many aquariums aim to keep nitrate under about 20 to 40 ppm with regular partial water changes.

Next step

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