Know your nitrite

High nitrite levels can be critical to the health of your fish in both ponds and aquariums. But what is it and what causes it?

What is nitrite and what causes it?

Nitrite (NO2) – not to be confused with nitrate (although your spellcheck may think otherwise) is a highly toxic pollutant which is produced by bacteria.

What is the nitrogen cycle?

Beneficial filter bacteria oxidise deadly ammonia (NH4) from decomposing waste and turn it to nitrite. Other bacteria then convert the nitrite to nitrate (NO3). So it goes from ammonia to nitrite to nitrate. Ammonia being the most deadly and nitrate being the least.

What causes nitrites to build up?

Nitrite levels rise when there’s too much polution for the positive bacteria to deal with.

What causes this polution that leads to high nitrite?

In ponds and aquariums alike, overfeeding and overstocking can lead to high nitrite levels, but an aquarium also needs to “mature” in order for the friendly bacteria to thrive. This is known as “new tank syndrome.

How does it harm the fish?

Nitrite gets into the fish’s blood stream through their gills and oxidises the iron in the blood which means they can’t carry oxygen around the blood. Exposure to nitrite can also affect their immune system against conditions like white spot and bacterial infections – if the nitrite hasn’t already killed them.

What symptoms might the fish show?

Because they can’t get oxygen they will gasp at the surface of the water and flap their gills.

What level should I aim for?

The nitrite level should always be zero.

What should I do if I detect nitrite?

Prevention is better than cure. The first thing you need to do is discover the cause so you can make sure it doesn’t happen again. Then, make a large water change to dilute the concentration of nitrite in the water.

What can I add to make nitrite less toxic?

Aquarium salt (sodium chloride) – not marine salt – is a traditional way of reducing the toxicity by providing chloride which reduces both the methaemoglobinemia (which prevents the oxygen from travelling round the blood) and the toxicity of nitrite in the blood.

Pond test kits are available for nitrite removal and a wide range of aquarium test kits are also available.

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Georgina is a member of the Swell UK marketing team and has been keeping tropical fish for a number of years now. Her favourite fish being the stunning, male Siamese Fighting Fish. She is also looking to expand her existing collection to include keeping saltwater fish as well. Her other pets include Bengal cat, Walter, and Labrador and Rottweiler cross, Presley.


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