During a reef tank cycle, ammonia rises first, then falls as nitrite rises; nitrite then falls as nitrate builds. Both ammonia and nitrite are toxic to livestock. The cycle is finished when both read zero and nitrate is present. Test both every two or three days and log them, so you can see which stage you’re in.
The three stages
Bulk Reef Supply describes the chain: waste and food release ammonia; one group of bacteria turns it into nitrite; a second turns nitrite into nitrate.
| Stage | Ammonia | Nitrite | Nitrate |
|---|---|---|---|
| Early | Rising | Zero or low | Zero |
| Middle | Falling | Rising | Starting to appear |
| Late | Zero | Falling | Rising |
| Cycled | Zero | Zero | Present |
What to test, and how
Test ammonia and nitrite every two or three days during the cycle, and nitrate weekly. Use kits made for saltwater; some freshwater ammonia kits don’t work in seawater. Note which kit you used.
Bulk Reef Supply’s cycling guide treats the tank as cycled when ammonia has read zero for two or three weeks, nitrite is zero and nitrate is present.
If ammonia comes back
A rise in ammonia after the cycle usually means too much was added at once: several fish together, heavy feeding or something that died. Bulk Reef Supply warns that adding too many fish at once risks an ammonia spike that can wipe out the bacteria.
Stop feeding for a day, remove anything dead, and test daily. A water change dilutes it while the bacteria catch up.
Common questions
What ammonia level is safe for fish?
Zero is the target in an established tank. Any reading in a tank with fish needs action.
Why is nitrite not dropping?
The second group of bacteria grows more slowly. Keep the ammonia source steady and wait.
Should I do water changes during the cycle?
Usually not until it finishes, unless your product says otherwise. Then do one to lower nitrate.
Sources and limits
Last checked 13 October 2026. This guide brings together the published advice below. Product instructions come first, and an experienced local shop or club can help with anything specific to your tank.
- Bulk Reef Supply: the nitrogen cycle in your aquarium
- Bulk Reef Supply: cycling your saltwater aquarium
- Randy Holmes-Farley, Reefkeeping: Reef Aquarium Water Parameters
Keep going
Log ammonia and nitrite · Free Reef Companion · All calculators