Guide
What to do when nitrite spikes
Nitrite is toxic to fish. Here’s how to lower it quickly and get your tank back on track.
- Water change to dilute nitrite; reduce or skip feeding.
- Don’t rinse or replace filter media in tap water.
- Keep testing until nitrite (and ammonia) stay at zero.
Why nitrite spikes
Nitrite is the middle step of the nitrogen cycle: bacteria turn ammonia into nitrite, and other bacteria turn nitrite into nitrate. During cycling, you often see a nitrite spike after ammonia starts to fall. In an established tank, a nitrite spike usually means the cycle was disrupted.
What to do now
- Test — Confirm ammonia and nitrite. Log results so you can track progress.
- Water change — A large partial change (30–50%) with dechlorinated water dilutes nitrite. Repeat as needed until nitrite stays at zero.
- Reduce feeding — Less food = less waste. Skip a day or two or feed very lightly.
- Don’t touch the filter — Filter media holds the bacteria that consume nitrite. Rinsing in tap water can prolong the spike.
Keep learning
Stable aquariums come from consistent testing and patient stocking. Continue with our water parameters guide, how to cycle a tank, and combining fish safely. For logging and strip scans, see App-aquatic.
Log parameters, scan strips offline, and run stocking checks with App-aquatic.
Get the free appWhy nitrite spikes?
Nitrite is the middle step of the nitrogen cycle : bacteria turn ammonia into nitrite, and other bacteria turn nitrite into nitrate. During cycling , you often see a nitrite spike after ammonia starts to fall. In an established tank, a nitrite spike usually means the cycle was disrupted.
How does regular testing help?
Regular testing lets you catch ammonia, nitrite, and nitrate shifts before fish show stress. Log results over time to see trends, not single snapshots.
Where can I track parameters and strips?
App-aquatic logs water tests, supports offline test strip scanning, species ID, and stocking checks — with reminders so maintenance does not slip.
More guides · Ammonia spike · How to cycle · Water parameters · Get the app