Nitrite is less dangerous than ammonia.

Both cause serious harm at low concentrations. Nitrite is often the longer phase during cycling, so fish are exposed for longer.

compoundNO₂⁻mg/L

Nitrite

Nitrite appears during cycling and whenever the second stage of the nitrogen cycle cannot keep up with the first. Unlike ammonia, its toxicity does not depend strongly on pH; it depends on chloride, because nitrite competes with chloride for the same gill uptake pathway.

In one sentence

Nitrite is the intermediate product of the nitrogen cycle. It is taken up across the gills and oxidises haemoglobin to methaemoglobin, which cannot carry oxygen — so a fish in nitrite suffocates in fully oxygenated water.

How to read a measurement

Bands are guidance for interpreting a reading, not thresholds with sharp edges. Sensitivity varies by species and by how long the exposure lasts.

Band	Range	Verdict	What it means
Not detected	≤ 0.05	target	Normal for a cycled tank.
Trace	0.05–0.25	act	The filter is not keeping up. Water change and reduce feeding.
Elevated	0.25–1	urgent	Large water change now.
High	≥ 1	urgent	Emergency. Large water changes, aeration, and no feeding.

Why fish gasp in nitrite

Nitrite converts haemoglobin to methaemoglobin, which is brown and cannot bind oxygen. The blood becomes progressively less able to carry oxygen regardless of how much is dissolved in the water, which is why affected fish gasp at the surface even in a well-aerated tank. In severe cases the gills and blood take on a visibly brown cast, hence the aquaculture name 'brown blood disease'.

The chloride relationship

Nitrite enters the fish through the chloride uptake pathway on the gills, so raising the chloride concentration in the water competitively reduces nitrite uptake. This is the reason salt is recommended during a nitrite spike, and it is one of the few uses of aquarium salt with a clear mechanism behind it.

It is not a fix. It protects the fish while the underlying problem — an under-developed or overloaded filter — is corrected. It is also not appropriate for every tank: some scaleless fish and many plants respond badly to chloride additions, and invertebrates are sensitive.

How it is measured

Liquid test kit
Test strips

Typical aquarium range

Typical: 0–0 mg/L

Common misconceptions

Often said	What is actually the case
Nitrite is less dangerous than ammonia.	Both cause serious harm at low concentrations. Nitrite is often the longer phase during cycling, so fish are exposed for longer.