Nitrite Poisoning in Aquarium Fish

Nitrite value testing
armcreation / Getty Images

Nitrite poisoning follows closely on the heels of ammonia as a major killer of aquarium fish. Just when you think you are home free after losing half your fish to ammonia poisoning, the nitrites rise and put your fish at risk again. Anytime ammonia levels are elevated, elevated nitrites will soon follow. To avoid nitrite poisoning, test when setting up a new tank, when adding new fish to an established tank, when the filter fails due to power or mechanical failure, and when medicating sick fish.

  • Names: Brown Blood Disease, Nitrite Poisoning
  • Disease Type: Environmental
  • Cause / Organism: Nitrite


  • Fish gasp for breath at the water surface
  • Fish hang near water outlets
  • Fish is listless
  • Tan or brown gills
  • Rapid gill movement

Also known as 'brown blood disease' because the blood turns brown from an increase of methemoglobin. However, methemoglobin causes a more serious problem than changing the color of the blood. It renders the blood unable to carry oxygen, and the fish can literally suffocate even though there is ample oxygen present in the water.

Different species of fish tolerate different levels of nitrite. Some fish may simply be listless, while others may die suddenly with no obvious signs of illness. Common symptoms include gasping at the surface of the water, hanging near water outlets, rapid gill movement, and a change in gill color from tan to dark brown.

Fish that are exposed to even low levels of nitrite for long periods of time suffer damage to their immune system and are prone to secondary diseases, such as ich, fin rot, and bacterial infections. As methemoglobin levels increase, damage occurs to the liver, gills, and blood cells. If untreated, affected fish eventually die from lack of oxygen and/or secondary diseases.

Treatment of Nitrite Poisoning

  • Large water change
  • Add salt, preferably chlorine salt
  • Reduce feeding
  • Increase aeration

The addition of a half ounce of salt per gallon of water will prevent methemoglobin from building up. Chlorine salt is preferable; however, any aquarium salt is better than no salt at all. Aeration should be increased to provide ample oxygen saturation in the water. Feedings should be reduced and no new fish should be added to the tank until its ammonia and nitrite levels have fallen to zero.

Nitrite is lethal at much lower levels than ammonia. Therefore it is critical to continue daily testing and treatment until the nitrite falls to zero.

Prevention Tips

  • Stock new tanks slowly
  • Feed sparingly and remove uneaten food
  • Change water regularly
  • Test water regularly to catch problems early

The key to eliminating fish death is to avoid extreme spikes and prolonged elevation of nitrites. When starting a new tank, add only a couple of fish initially and do not add more until the tank is completely cycled. In an established tank, only add a couple of new fish at a time and avoid overstocking.

Feed fish small quantities of foods and remove any food not consumed in five minutes. Clean the tank weekly, taking care to remove any dead plants or other debris. Perform a partial water change at least every other week, more often in small heavily stocked tanks. Always test the water for nitrite after an ammonia spike has occurred as there will be a nitrite increase later.

If you suspect your pet is sick, call your vet immediately. For health-related questions, always consult your veterinarian, as they have examined your pet, know the pet's health history, and can make the best recommendations for your pet.