The nitrogen cycle in a new tank
Fish waste and leftover food release ammonia, which burns gills. One group of bacteria turns ammonia into nitrite, which is also toxic, and a second group turns nitrite into nitrate, which is far less harmful and is removed by water changes. A new filter has almost none of these bacteria.
Merck describes new tank syndrome, in which fish are poisoned by ammonia and nitrite, as usually occurring within the first 6 weeks, and notes that the biofilter can take up to 8 weeks to establish.
| Stage | Ammonia | Nitrite | Nitrate | What to do |
|---|---|---|---|---|
| 1. Ammonia | High | 0 | 0 | Keep ammonia available; wait for nitrite |
| 2. Nitrite | Falling | Rising | Starting | Keep dosing; nitrite often peaks high |
| 3. Nearly there | 0 | Falling | Rising | Keep dosing until nitrite clears |
| Cycled | 0 | 0 | Present | Water change, then add fish slowly |
Fishless cycling (recommended)
Merck recommends fishless cycling: add ammonia to a concentration of 1–5 mg/L and test for ammonia, nitrite and nitrate. Once no ammonia or nitrite is present, fish can be added. Because no fish are exposed, you can let levels run high without harm.
Before stocking, confirm by dosing ammonia once more. If ammonia and nitrite both read 0 within 24 hours, the filter can cope. Do a large water change to bring nitrate down, then add fish in small groups.
If fish are already in the tank
Merck warns that stocking a new tank slowly with fish to cycle it can cause ammonia and nitrite spikes and can be inhumane. If fish are already in, test daily, feed sparingly, do partial water changes whenever ammonia or nitrite shows, and use a water conditioner that binds ammonia. The tracker flags any reading of 0.5 mg/L or more.
Signs of ammonia or nitrite poisoning
When the cycle stalls
- Chlorine or chloramine in untreated tap water kills bacteria. Always dechlorinate.
- Very low KH or pH slows nitrifying bacteria. Check with the GH & KH Calculator.
- Too much ammonia (well above 5 mg/L) can slow the second stage. Do a water change and dose less.
- Cold water slows bacteria; tropical temperatures speed the cycle.
- Seeded media from a healthy tank can shorten cycling dramatically.
