The Nitrogen Cycle
Your filter doesn’t just move water – it houses bacteria that turn deadly fish waste into something your tank can cope with. This process is the nitrogen cycle. Once it’s running properly, it quietly protects your fish every hour of every day.
In a healthy aquarium, you should never see ammonia or nitrite on a test kit. That’s not luck – that’s the cycle doing its job in the background.
Need help reading ammonia, nitrite and nitrate?
Use the Fish Tank Cycling Tool & Aquarium Cycle Checker to check your aquarium cycle, understand whether your tank is cycled, and see what to do next before adding fish.
Produced by fish waste, uneaten food and anything rotting in the tank. Even 0.25 ppm can stress fish; above 1.0 ppm is often dangerous.
- Comes from fish poop & rotting food
- First thing to appear in a new tank
- The cycle starts when ammonia builds up
One group of bacteria eats ammonia and produces nitrite. Nitrite is still toxic – it stops blood carrying oxygen properly, even if the water looks “clean”.
- Appears after the ammonia peak
- Should always read 0 ppm in a mature tank
- A mid-cycle “nitrite spike” is completely normal
Final product of the cycle. Much less toxic, but still needs controlling with water changes and plants.
- Aim for < 40 ppm for most fish
- Lower (< 20 ppm) for sensitive species
- Removed by water changes & live plants
Best and safest method. You grow the bacteria first so fish never see harmful levels of ammonia or nitrite.
Basic steps
- Fill the tank, add heater and filter, and run everything 24/7.
- Add bottled ammonia to about 2 ppm.
- Test regularly for ammonia, nitrite and nitrate.
- When ammonia hits 0, top it back up to 2 ppm.
- Wait for nitrite to rise, then later drop back to 0.
- The tank is considered cycled when it processes 2 ppm ammonia to 0 ppm ammonia and 0 ppm nitrite within 24 hours.
- Do a 50–70% water change to bring nitrate down, then start adding fish slowly.
Ways to speed it up
- Use media or a sponge from an established, healthy tank.
- Add bottled bacteria (e.g. Stability, SafeStart, etc.).
- Keep temperature around 26–28 °C.
- Never wash filter media in tap water – always use tank water.
Used when fish are already in the tank and you discover the filter isn’t cycled. It can work, but it’s stressful for the fish and needs a lot of testing and water changes.
Rules to protect fish
- Keep ammonia < 0.25 ppm.
- Keep nitrite < 0.25 ppm.
- Test daily; change water as often as needed to stay under those numbers.
- Use a conditioner that detoxifies ammonia/nitrite (e.g. Seachem Prime).
- Feed very lightly – less food = less waste.
- Add bottled bacteria to help the filter catch up.
Typical timeline
- Cycle length: anywhere from 10 days to 6 weeks.
- Early: ammonia rises first.
- Middle: ammonia drops, nitrite spikes.
- Late: ammonia 0, nitrite 0, nitrates rising – the tank is cycled.
- Ammonia: 0 ppm
- Nitrite: 0 ppm
- Nitrate: present and slowly rising
- Filter has been running continuously (no long power-offs)
- The tank can process a measured dose of ammonia within 24 hours
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“The water holds the cycle.”
Most bacteria live in the filter media, not the water. -
“Clear water means safe water.”
Ammonia and nitrite are invisible – you have to test. -
“You have to use lots of chemicals.”
Bacteria, ammonia and time are enough; bottled products just speed things up. -
“Fish will just ‘tough it out’.”
Fish-in cycling can permanently damage gills and shorten lifespans.