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Nitrite poisoning (brown blood disease)

Nitrite is the intermediate product of the nitrogen cycle, produced from ammonia by one group of bacteria and consumed by another. In a tank that is part-way through establishing, the first group arrives before the second, and nitrite accumulates. It is taken up at the gill through the chloride transport pathway, and once in the blood it oxidises the iron in haemoglobin, producing methaemoglobin — brown rather than red, and unable to bind oxygen. Hence the name, and hence the characteristic picture of a fish gasping in well-aerated water. The competitive uptake with chloride is the mechanistically important detail: raising chloride reduces nitrite uptake, which is a genuine, mechanism-based intervention rather than a folk remedy.

Short answer

Nitrite poisoning suffocates a fish in water that is full of oxygen. Nitrite is absorbed across the gill and converts haemoglobin into methaemoglobin, which cannot carry oxygen — so the fish gasps while the dissolved oxygen level is perfectly adequate. Chloride in the water competes with nitrite for the same uptake pathway, which is why it is protective.

What you can and cannot establish yourself

Suggestive by eye, not conclusive

You can see it, but several conditions look like this. Work through the alternatives below before deciding what you are treating.

Cause
Nitrite taken up across the gill, which oxidises haemoglobin into a form that cannot carry oxygen
Spread between fish
Not transmissible
Urgency
Act now
Affects
fish, shrimp, snail, crayfish, crab, other-invertebrate
Confidence
high confidence

Signs

  • Gasping at the surface despite good aeration — the defining picture
  • Rapid gill movement
  • Gills appearing brown or tan rather than red
  • Lethargy and loss of appetite
  • Fish hanging near the filter outflow
  • All fish affected at once
  • Occurring typically a few weeks into a new tank, after the ammonia phase

Do this first: the environmental response

Water quality is the cause, the contributing factor or the deciding variable in the great majority of aquarium health problems. These steps come before any treatment decision, and for several conditions they are the entire treatment.

  1. Large water change immediately, dechlorinated and temperature-matched.
  2. Stop feeding until nitrite reads zero.
  3. Increase aeration. It does not fix the underlying problem — the blood cannot carry the oxygen — but it maximises what the remaining functional haemoglobin can pick up.
  4. Adding chloride to the water reduces nitrite uptake at the gill by direct competition. This is the one genuinely mechanism-based chemical intervention in this condition, and it is protective rather than curative. It must be weighed against plants and invertebrates in the tank, several of which tolerate added chloride poorly.
  5. Test ammonia as well; the two commonly occur together.
  6. Keep testing daily until stable — nitrite can rebound as the filter continues to develop.

What else it could be

Every entry names the observation that separates it from this record. Work through these before deciding what you are treating — the most expensive mistake in fish health is confident treatment of the wrong thing.

Non-disease explanations — check these first

  • Ammonia poisoning

    Husbandry causevery common

    How to tell it apart: Nearly identical presentation and frequently present at the same time. Test both. The distinction matters because raising chloride helps against nitrite and does nothing against ammonia, and because pH manipulation is dangerous with ammonia present.

  • Low dissolved oxygen

    Husbandry causevery common

    How to tell it apart: Both produce gasping. The distinguishing observation is that nitrite poisoning continues despite vigorous aeration, because the problem is the blood's carrying capacity rather than the water's oxygen content.

Other conditions

What makes it more likely

Each entry states the mechanism, not just the association — an association without a mechanism is not something you can act on.

  • Low chloride in the water. Nitrite is taken up through the chloride pathway at the gill. In soft, low-chloride water there is less competition and uptake is greater, which is why the same nitrite level is more dangerous in soft water.
  • A newly cycling tank. The nitrite-consuming bacteria establish after the ammonia-consuming ones, so nitrite accumulates during a predictable window.
  • Recent filter cleaning or medication. Both can remove or suppress the bacteria that consume nitrite.

Treatment categories

AquariumHQ names classes of response and never doses. Product concentrations differ between markets and formulations, several actives are temperature- and pH-dependent, and the invertebrate and plant tolerances that decide whether the rest of the tank survives are not printed on the label. Where a category puts a chemical in the water, what it can damage is stated with it.

Water-change regime

Dilution reduces the concentration immediately and is the primary response.

Salt

Adds a chemical to the water

Chloride competes with nitrite for uptake at the gill, reducing the amount absorbed. This is a well-established mechanism and is protective while the filter establishes, not a cure for the exposure already sustained. AquariumHQ gives no concentration: the appropriate level depends on the species present, the plants, the invertebrates and the existing chloride content of the water, and figures circulating in the hobby are frequently transcribed from aquaculture contexts with different stock and different water.

What it can damage: Many aquarium plants tolerate added chloride poorly, ornamental shrimp and snails are sensitive, and several soft-water species including a number of catfish and loaches do badly. In a planted or invertebrate tank this intervention may cost more than it saves.

Do not use

  • Heavily planted tanks
  • Tanks with ornamental shrimp or snails
  • Species with poor tolerance of added chloride

Increase aeration

Maximises oxygen loading of the haemoglobin that is still functional.

Correct the husbandry

Reducing feeding and stocking, and allowing the filter to establish, is what ends the episode.

Isolation and quarantine

  • The water is the problem. Isolation helps only if the destination water is genuinely better.

When a vet is the right call

  • Severe damage with welfare decisions to make
  • Continued losses after nitrite has returned to zero
  • Secondary infection following gill damage

AquariumHQ describes what is commonly reported and what husbandry correction is usually indicated. It does not diagnose and is not a substitute for an aquatic veterinarian. Many treatments are given to healthy fish suffering from a water-quality problem, which makes the outcome worse rather than better.

Prevention

  • Cycle the tank fully before stocking, and expect a nitrite phase after the ammonia phase.
  • Stock gradually.
  • Do not clean filter media in untreated tap water.
  • Test through the whole establishment period, not only at the start.
  • Be aware that soft, low-chloride water makes a given nitrite reading more dangerous.

What AquariumHQ does not know

Published rather than hidden. A health page that lists no limitations has not had its limitations looked for.

  • Species-level sensitivity varies considerably and is documented mainly for aquaculture species. AquariumHQ publishes no per-species thresholds.
  • The chloride-to-nitrite ratio used in aquaculture is derived from production species in defined water, and AquariumHQ does not transfer it to ornamental aquaria as a rule of thumb.

Evidence

Each claim is shown with what kind of support stands behind it and how strongly we hold it. Those are separate axes: a measurement held tentatively and an editorial position held firmly are different things.

Observed / measuredhigh confidence

Nitrite oxidises haemoglobin to methaemoglobin, which cannot transport oxygen, producing functional hypoxia in well-oxygenated water

  • MSD/Merck Veterinary Manual — fish chapters — Fish — nitrite toxicity and methaemoglobinaemia
  • Noga, E.J. — Fish Disease: Diagnosis and Treatment (2nd edn, 2010) — Water quality — nitrite

Last verified 2026-08-11

Observed / measuredhigh confidence

Nitrite is taken up at the gill via the chloride transport pathway; increased ambient chloride competitively reduces uptake and is protective

  • MSD/Merck Veterinary Manual — fish chapters — Fish — nitrite toxicity, prevention
  • UF/IFAS Extension — fish health and aquaculture publications — Water quality — nitrite and chloride

Last verified 2026-08-11

Derived from other datahigh confidence

Increasing aeration does not resolve the underlying defect, because the limitation is the blood's oxygen-carrying capacity rather than dissolved oxygen availability

  • AquariumHQ husbandry synthesis — AquariumHQ derivation from the methaemoglobin mechanism

Last verified 2026-08-11