Frequently asked questions on nitrite


Nitrite is a toxic pollutant and high levels can be deadly for your fish. Matt Clarke explains what causes nitrite to rise, and what you can do to save your fish in a water quality crisis...

What is nitrite?

Nitrite (NO2) is a highly toxic pollutant which is produced by bacteria during the breakdown of fish wastes and other organic materials through a process called nitrification.

Beneficial filter bacteria called Nitrosomonas, oxidise deadly ammonia (NH4) from decomposing materials and fish wastes to nitrite, which is slightly less toxic to fish. Other bacteria, like Nitrospira and Nitrobacter, then convert the nitrite to nitrate (NO3), which is only mildly toxic by comparison. It's very important to test your water regularly for signs of ammonia, nitrite and nitrate.

Why does nitrite build up?

High nitrite levels arise because there's too much pollution being produced for the beneficial bacteria to cope with.

Filter bacteria are present in proportion to the amount of pollution entering the tank and take time to grow to levels where they can handle all of the pollution being produced by the fish.

While they're building-up their numbers in an attempt to consume all of the food available to them, the water will be polluted with ammonia and nitrite, which is stressful to the fish. When there are plenty of bacteria and a constant amount of waste gets added each day, the ammonia and nitrite level should stay at zero.

What causes high nitrite?

Overfeeding and overstocking can lead to high nitrite levels, but incorrect filter maintenance and new tank syndrome are perhaps the most common cause.

New filters (and media which has been washed in tapwater or replaced with new) contain few bacteria and aren't capable of removing much pollution. It generally takes the bacteria 4-6 weeks to build-up, or mature, depending on the temperature and chemistry of the water.

Only wash filter media in old tank water, not under the tap, and always replace media bit by bit, not all at the same time.

To speed-up the growth of bacteria in new filters you can transfer some media from an existing filter, or add a bacterial starter culture and food source.

If you've got a new tank, or filter, don't add too many fish, and don't add much food. The more pollution entering the tank the more dangerous the conditions will become for any fish present.

How does it harm the fish?

Nitrite enters the blood of the fish at the gills and can harm the fish in a number of different ways.

Most notably it oxidises the iron in their haemoglobin to produce a molecule called methaemoglobin, which unlike haemoglobin, can't carry oxygen around the blood. This process (called methaemoglobinemia) sometimes turns the blood brown and causes extreme breathing difficulty or even suffocation.

Nitrite also builds up in the blood causing poisoning, and may lead to liver, gill and blood cell damage.

Exposure to nitrite for long periods makes the immune system less effective and the fish may start to suffer from diseases, such as white spot and bacterial infections, like fin rot or ulcers, if they haven't already died from nitrite poisoning.

What symptoms might the fish show?

Due to the problems they experience in breathing normally, affected fish often gasp or hang at the surface where the oxygen content of the water is at its greatest. They'll usually move their gills more quickly, and may hold their fins close to their body. Although fish may show some signs of nitrite poisoning, you can't tell how much is present unless you test the water.

Is it toxic to all fish?

Different species of fish, and even different individuals of the same species, may have different tolerances to nitrite. Some species can reduce the rate at which it enters the blood through the gills, and it's toxicity is also affected by water chemistry. Therefore, not all of the fish in the tank or pond will fall ill or die as a result of a raised nitrite level.

What level should I aim for?

The nitrite level should always be zero, or as close to zero as you can get it. Under certain conditions, even relatively low nitrite levels of 0.25 mg/l may be enough to weaken sensitive species. Anything above 0.1 mg/l should be viewed as unacceptable and a potential cause of stress, although some fish might tolerate very high levels.

Check the form the nitrite result is given as. Some results need converting from nitrite-nitrogen.

What should I do if I detect nitrite?

Your first course of action upon finding nitrite should be to find out why the level is high and try to ensure that whatever caused it never happens again. Then, make a large (50% or more) partial water change to dilute the concentration of nitrite in the water.

Test daily and be prepared to make further water changes to reduce the levels.

Losses are always higher when the dissolved oxygen levels are lower, so boost the dissolved oxygen levels by adding an airstone, or by using a venturi. Keep a close eye on the fish to see if they develop any diseases.

What can I add to make nitrite less toxic to the fish?

Aquarium salt (sodium chloride) has long been used as an aid to reducing the toxicity of nitrite, because it has been shown to prevent methaemoglobinemia under certain conditions.

Some fishkeepers don't like using salt, but in my experience, when used at the correct dosage, it's never caused problems for any fish, even stereotypically salt-intolerant species. A fairly low level of salt can have a significant effect on reducing the toxicity of nitrite, so you don't need to add very much.

Research suggests a 10:1 dose (just 10mg/l of salt per 1mg/l nitrite) is effective for most freshwater species. The addition of salt for controlling disease or osmoregulatory problems does need higher doses!

Why does adding salt help?

The addition of salt provides chloride which reduces both the methaemoglobinemia and the toxicity of nitrite in the blood. However, interestingly, although the chloride has always been provided via sodium chloride, more recent research suggests that calcium chloride can work as well, if not better, because the additional calcium is also thought to decrease gill permeability, which prevents as much nitrite entering the blood.

For this reason, fishes kept in water with a higher calcium and chloride level, are usually less sensitive to nitrite than those in freshwater with a low chloride and calcium concentration - but many marines remain sensitive.

Theoretically, fish kept in warm, soft water (ie. Discus) are most at risk, but nitrite can and does kill fish in all water conditions.