The short answer
High nitrite means your tank is mid-cycle — the bacteria that convert nitrite into harmless nitrate haven’t fully established yet — or an established cycle has been disrupted. Nitrite is toxic to fish, so lower it with a water change and a detoxifying conditioner, keep testing, and give the bacteria time to catch up. Don’t add fish until it reads zero.
Why nitrite spikes
During cycling, ammonia is first converted to nitrite by one group of bacteria. A second group then converts nitrite to far-less-toxic nitrate — but it grows more slowly, so nitrite builds up in the meantime. You’ll often see ammonia at zero but nitrite high: that’s the cycle progressing normally.
An established tank can also spike if the filter was over-cleaned, went unpowered, or the stocking suddenly increased.
How to lower it fast
- Do a water change — 25–50% dilutes nitrite immediately. Repeat daily while it stays high. See how to do a water change.
- Dose a detoxifying conditioner such as Seachem Prime, which detoxifies nitrite as well as ammonia for a day or so. Browse our conditioner picks.
- Stop feeding for a day or two to reduce the waste feeding the spike.
- Don’t add fish or clean the filter while nitrite is elevated.
Letting the cycle finish
The permanent fix is a fully established cycle. Keep testing and doing changes until nitrite holds at zero on its own, which means the second bacteria group has caught up. A bacteria starter or seeded media speeds this along. For the full method, see how to cycle an aquarium and fishless cycling, and the water testing hub.