Nitrite in Your Aquarium

Nitrite value testing

Fish excrete ammonia through their gills as their nitrogenous waste product. The toxic ammonia is converted by beneficial bacteria in the aquarium to nitrite, which is also toxic to the fish. Fortunately, in an established aquarium that has completely cycled, other bacteria convert nitrite into harmless nitrate. However, in new aquariums where the bacteria population hasn’t grown adequately to detoxify the wastes from the fish in the aquarium, ammonia can build up to toxic levels quickly.

Nitrite poisoning follows closely on the heels of elevated 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 nitrite level rises and puts your fish at risk again. Anytime ammonia levels are elevated, elevated nitrite will soon follow. To avoid nitrite poisoning, test the water 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 are listless
  • Brown gills
  • Rapid gill movement

Nitrite poisoning is also known as “brown blood disease” because the blood turns brown from an increase of methemoglobin. However, methemoglobin causes a more serious problem than just 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 the normal pink 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 aquarium salt (sodium chloride) or a marine salt mix
  • Reduce feeding
  • Increase aeration

First, perform water changes with dechlorinated water to reduce the nitrite level. The addition of a half-ounce (1 tablespoon) of salt per gallon of water will prevent methemoglobin toxicity by blocking the nitrite absorption through the fish’s gills. Any aquarium salt or marine salt mix can be used. Do not use iodized table salt. 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 the ammonia and nitrite levels have fallen to zero. That will reduce the formation of excess ammonia, which will be converted to nitrite. It is critical to continue daily testing and treatment until the nitrite level 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 nitrite. When starting a new tank, add only a couple of fish initially and do not add more until the tank is completely cycled. That way the population of beneficial bacteria will grow as the level of ammonia produced by the fish increases. 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. The beneficial bacteria in the biofilter will eventually convert the nitrite into harmless nitrate, but that too should be removed by periodic partial water changes.

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