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 fish’s 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 nitrate. To be sure whether nitrate levels are toxic to your fish you will need to purchase a nitrate testing kit from your local aquatic specialist. You may need to research the appropriate nitrate level tolerable for your specific fish.

Most popular fish will be happy in nitrate levels less than 100 ppm, however more sensitive species will require a lower level otherwise they may succumb to nitrate poisoning - particularly if levels remain high. The resulting stress will leaves fish more susceptible to disease and inhibits their ability to reproduce. For all intents and purposes levels it best that levels stay below the 50ppm mark.

Fish with nitrate poisoning will often appear very sluggish. Their gills will be opening and closing rapidly and they will often be found swimming at the surface of the tank ‘gasping’ for air. Occasionally you will be able to notice brown or yellowish discolorations of the gills.
Higher or prolonged exposure to high levels of nitrate will show signs of loss of appetite, fish resting on the bottom, a bent or curled positioning of the body, crooked spine, uncontrolled swimming or swimming in circles, spasms or twitching. Usually at this point the fish is unlikely to survive!


The first and perhaps the easiest way to reduce nitrate 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 poop, rotting vegetation – that could be responsible for high nitrate levels from 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 from the water. However, sickly or dying plants will be contributing to the problem so make sure that they are removed on sight.

Native British Pond Plants

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