Tip:Quick answer: the nitrogen cycle is how helpful bacteria turn toxic fish waste (ammonia) into much safer nitrate. A new tank needs about 4 to 6 weeks to finish it.
This is a demo article used to test every content feature of the site. Delete it when you are done. A normal outside link such as the Wikipedia page on the nitrogen cycle should be rendered with nofollow by default, while a link you trust can pass credit, like this dofollow link. An internal link looks normal: read our quarantine guide.
What is the nitrogen cycle?
Fish produce ammonia through gills and waste. Bacteria in the filter convert ammonia to nitrite, and a second group converts nitrite to nitrate. Water changes remove the nitrate.
The three stages at a glance
| Stage | Compound | Danger level | What to do |
|---|---|---|---|
| 1 | Ammonia (NH3/NH4) | High | Test daily, do small water changes |
| 2 | Nitrite (NO2) | High | Keep changing water, feed lightly |
| 3 | Nitrate (NO3) | Low | Weekly water changes of 20 to 25% |
Watch: how the cycle works
Cycle setup checklist
Before you add fish
- Dechlorinate all tap water
- Run the filter 24 hours a day
- Add a bottle of beneficial bacteria
- Test ammonia, nitrite and nitrate every 2 days
- Add fish only when ammonia and nitrite read zero
Signs your tank is cycled
- Ammonia reads 0 ppm
- Nitrite reads 0 ppm
- Nitrate reads 5 to 40 ppm
Heads up:Never add many fish at once to a new tank. A sudden waste spike can crash the cycle.
Frequently asked questions
Frequently asked questions
How long does the nitrogen cycle take?
Can I cycle a tank with fish in it?
Why is my ammonia not dropping?
DEMO: Ammonia, nitrite and nitrate during cycling
Values in ppm
| Week 1 | 2 ppm |
|---|---|
| Week 2 | 4 ppm |
| Week 3 | 3 ppm |
| Week 4 | 1 ppm |
| Week 5 | 0.5 ppm |
| Week 6 | 0 ppm |
DEMO: Share of cycling methods people use
- Fishless with ammonia45%
- Seeded media30%
- Fish-in25%
DEMO: Typical filter flow rate
- Sponge filter80 gph
- Hang-on-back200 gph
- Canister350 gph
