High nitrite in an aquarium: what to do right now
Updated on August 26, 2026 · ~6 min read
Nitrite in an aquarium must read 0 ppm: above zero it stops fish blood from carrying oxygen, so the fish suffocate even in well oxygenated water. You act with repeated 30-50% water changes, food suspended and no new arrivals until the value is back to zero.
If the nitrite test reads anything above zero, the tank has a problem happening right now. This is not a parameter to "keep an eye on": it is the second step of the nitrogen cycle and the correct value is 0 ppm, in freshwater as in marine.
Why it kills, and why an air pump is not enough
Nitrite (NO₂⁻) crosses the gills and enters the blood, where it binds to haemoglobin and turns it into methaemoglobin: a molecule that no longer carries oxygen. The result is a fish that suffocates in fully oxygenated water.
That detail explains both the most typical symptom and the first mistake. The fish sits at the surface with its mouth open and breathes fast, so it looks like an oxygen problem — and the instinctive reaction is to add an air pump. The air pump does no harm and helps a little, but it does not touch the cause: the oxygen is already there, it is the blood that cannot carry it. The only thing that genuinely helps is lowering the nitrite.
Symptoms, in the order they appear: rapid breathing, fish at the surface with an open mouth, listlessness, gills turning brownish, loss of appetite, scattered deaths days apart. In acute cases the blood takes on a brown colour visible in the gills.
What to do in the next few hours
1. A 30-50% water change. Water at the same temperature, treated with a dechlorinator. It is the only measure that lowers the value quickly, and you repeat it every day until the test reads zero again. Changing water every day for a week is far better than leaving the fish in nitrite.
2. Suspend feeding for 24-48 hours, then resume with reduced amounts. Less food means less ammonia, and so less nitrite produced downstream.
3. A pinch of salt, if the tank allows it. This is the only "trick" with a real basis: chloride competes with nitrite for uptake at the gills and reduces absorption. We are talking about 1-3 g of non-iodised salt per litre, dissolved separately and added slowly. ⚠️ Do not do it in tanks with delicate plants, shrimp or salt-sensitive fish (many Corydoras, many loricariids): in those cases the remedy can cost more than the problem.
4. No new arrivals, no medication, no fertiliser. And no new fish until the value has been at zero for several days.
5. More surface movement. It does not solve anything, but a fish in respiratory distress feels somewhat better. If CO₂ is running, shut it off.
Why it appeared
New tank, less than two months old. This is the most common and the most predictable case: it is the normal phase of the nitrogen cycle, the longest one. There is no mistake to correct, there is waiting to do and damage to limit. If there are fish in the tank, this is new tank syndrome.
A mature tank in which nitrite reappears. Here it is a fault, and the cause is one of these:
- The filter was washed under the tap, or all the filter media was replaced at once. Most of the bacteria live in there.
- The filter stopped for a few hours — a power cut, a clogged pump. Nitrifying bacteria are aerobic: without flow they die quickly.
- A medication or an antibiotic hit the useful bacteria too. It happens with almost every broad-spectrum antibacterial.
- Too many fish added at once. The colony is sized for the load it knows; doubling it in an afternoon puts it into debt.
- A dead animal out of sight, or a substrate cleaning after months, releasing a load of ammonia beyond the filter's capacity.
If nitrite appears in a mature tank with none of these causes, the next suspect is the test itself: nitrite reagents are among the first to degrade, and an old bottle gives false readings in both directions.
How long it takes
In a new tank, one to three weeks for the second group of bacteria to grow enough to process it. It is the slowest phase of the cycle and no product shortens it — at best bottled bacteria, which sometimes help a great deal and sometimes not at all.
In a mature tank whose filter was damaged, recovery is faster: a few days, if part of the colony survived.
The thing not to do in either case is the one that feels most sensible: strip it all down and start again. The bacteria live in the sponges and in the substrate. Emptying the tank halfway through a cycle wipes out weeks of work and takes you back to the start.
What not to do
- Do not use "nitrite removers" in place of a water change: none of them work as promised, and some interfere with the test.
- Do not change the filter media right now: it is the one place where the bacteria are growing.
- Do not trust test strips for nitrite: their sensitivity is inadequate precisely in the low part of the scale, which is the part that matters.
- Do not raise the temperature to speed up the cycle if fish are already suffering: warmer means less dissolved oxygen, and these fish already have an oxygen problem.
How to measure it without getting it wrong
The nitrite kit is one of the easiest to misread, for three reasons.
The reagents degrade sooner than the others. A bottle opened a couple of years ago can read zero on a tank sitting at 1 ppm — and since zero is the result you are hoping for, nobody questions it. Faced with symptomatic fish and a test at zero, the first suspect is the date on the bottle.
The colour scale is compressed at the bottom. Between 0 and 0.25 ppm there is one shade of pale pink. Read it in daylight, against a white background, looking down through the vial rather than from the side: from the side the water path is longer and the colour looks stronger.
Test strips are not enough. On nitrite their sensitivity is inadequate exactly in the low part of the scale, which is where you decide whether to act.
When in doubt, a water sample taken to a shop and measured with a different kit is worth more than the same measurement repeated twice with your own bottle.
How Gurglee does it
In Gurglee nitrite has zero as its reference and the tank flags it as soon as it drifts. What changes the decision, though, is seeing the effect of a water change before doing it: on the parameter's page the app works out where the value would land with the percentage you are considering, on that tank's volume, and it also tells you which percentage it would take to get back in range — so you know whether one change is enough or whether it will take two days. There is also the tank doctor, which answers by reading that tank's actual readings rather than in general terms.
Related guides
Ammonia in an aquarium: why it is the most dangerous
What ammonia is in an aquarium, why it kills, where it comes from and what to do when the test detects it. With the role of pH and temperature in its toxicity.
The aquarium nitrogen cycle: what it is and how long it takes
What the nitrogen cycle is, how long it really takes and how to tell when a tank is ready for fish. With the values to check week by week.
New tank syndrome: symptoms and what to do
Fish in a new tank are sick or dying: how to recognise new tank syndrome, what to test straight away and how to limit the damage.
Fish gasping at the surface: what to check
A fish sitting at the surface with its mouth open has an oxygen or a gill problem: the seven causes in order of likelihood and what to do in the first few minutes.