Nitrites (& Fish)

Fish which are kept by enthusiasts are usually held in a closed body of water such as a garden pond or fish tank.

Unfortunately, these relatively small volumes can create something of an ‘un-natural’ environment – often unsustainable without human intervention. This is because toxins from the fish, uneaten fish food and pollution from the general environment can – over time – build up in concentrations which are harmful to your fishes health.

In an open system of water such as the ocean or a river, levels of toxicity can be diluted but the introduction of new or fresh water or they can be absorbed safely by aquatic plants, algae and bacteria as part of their normal metabolic processes.

One of the more problematical toxins that are found within a closed system is nitrite. To be sure whether nitrite levels are toxic to your fish you will need to purchase a nitrite testing kit from your local aquaticspecialist.

You may need to research the appropriate nitrite level tolerable for your specific fish. Most popular fish will be happy in nitrite levels less than 20 ppm, however more sensitive species will require a lower level otherwise they may succumb to nitrite poisoning.


Nitrite is formed when Nitrosomonas sp. bacteria oxidise ammonia. Although it is less toxic than ammonia, elevated levels will still present a threat to fish health. Prolonged exposure at low levels can lead to stress and is often associated with stress-related disease such as bacterial ulcers and fin-rot. At high levels, skin and gillepithelia can be damaged and opportunistic bacteria and parasites may take advantage of stressed fish.

The main danger is from nitrite being actively transported across the gills and into the fish’s bloodstream where it oxidises normal haemoglobin into methemoglobin. Normal haemoglobin picks up oxygen at the gills and transports it to the body tissues where it is exchanged for carbon dioxide. Methemoglobincannot transport oxygen and therefore – in acute cases – fish will be effectively asphyxiated.
At low levels of Nitrite concentrations you may find fish rubbing against solid objects. As levels increase fish will become lethargic, but may still swim up to feed. If the fish is suffering from nitrite poisoning, the gills will change from a healthy pinkish/red to a pale tan to dark brown in colour. The fish may also show signs of respiratory distress, i.e gasping at the water surface or hanging around water inlets.


The first and perhaps the easiest way to reduce nitrite levels is to perform a partial water change of no more than 25%. Make sure that the temperature of the new water is as approximate to the contaminated water as possible and add a suitable dechlorinator if your new water is obtained directly from the mains water supply. It is advisable as part of your normal maintenance to perform partial water changes once every 2 – 4 weeks.
With an aquarium you could consider using a siphon gravel cleaner to perform your water changes as you will be able to remove any natural waste products – uneaten food, fish poo, rotting vegetation – that could be responsible for high nitrite levelsfrom the bottom of the tank at the same time.

Look at the amount of food that you are feeding your fish. Overfeeding is an easy and quick way to spoil your water quality. Always use a good quality food and feed no more than you fish will eat within 60 seconds. If any food is left after feeding – remove it!

Keep your filters in tip top condition by following the manufactures maintenance instructions.

Consider adding live plants to your pond/aquarium as a natural way to remove nitrites form the water. However, sickly or dying plants will be contributing to the problem so make sure that they are removed on sight.


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