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SESK: Stuff Everyone Should Know · sesk.info/fish-nitrite-brown-blood

Why is nitrite dangerous to fish?

It stops the blood from carrying oxygen, a problem called brown blood disease.

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  1. It makes the water too acidic
  2. It stops the blood from carrying oxygen ✔
  3. It clouds the water so fish cannot see

Why: Nitrite turns hemoglobin into methemoglobin, which cannot deliver oxygen. The gills can turn chocolate brown, and fish gasp at the surface.

Huh, didn't know that: Fish with nitrite poisoning often crowd where there is more oxygen or gulp at the surface.

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What do you call a fish that won't share? Shellfish.
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References

Answer: It stops the blood from carrying oxygen. Nitrite turns hemoglobin into methemoglobin, which cannot deliver oxygen. The gills can turn chocolate brown, and fish gasp at the surface.

  1. Merck Veterinary Manual: Environmental Diseases of Aquatic Animals in Aquatic Systems www.merckvetmanual.com/exotic-and-laboratory-animals/aquatic-systems/environmental-diseases-of-aquatic-animals-in-aquatic-systems
    NO 2 forms methemoglobin when absorbed into the fish's bloodstream, a condition referred to as brown blood disease.
    Checked 2026-10-11.
  2. Merck Veterinary Manual: Environmental Diseases of Aquatic Animals in Aquatic Systems www.merckvetmanual.com/exotic-and-laboratory-animals/aquatic-systems/environmental-diseases-of-aquatic-animals-in-aquatic-systems
    Behaviorally, fish affected by nitrite toxicity will seek oxygen, congregating in oxygen rich areas or piping at the surface.
    Checked 2026-10-11.

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