Frequently asked questions on nitrate

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Nitrate is less of a problem in your aquarium than ammonia or nitrite, but it still shouldn't be ignored. Matt Clarke answers some of the most frequently asked questions on nitrate.

What is nitrate?

Nitrate (NO3 ) is a by-product of the breakdown of fish wastes by beneficial bacteria living in the filter. It encourages algal growth and may be toxic to some fish and invertebrates.

How can I tell how much is present in the tank?

You should test your water every week for signs of nitrate using a liquid or tablet test kit. Test your tapwater at the same time and compare the two.

If you are maintaining your tank properly, the two nitrate levels should be quite similar. If you're not, the nitrate level in your tank will be much higher.

Surely nitrate hasn't always been seen as a problem?

There's been a sneaking suspicion that high levels can cause problems for fish for many years. Research has failed to show that it is toxic to most species, except at high levels. However, fishkeeping experience suggests that it does actually cause a number of problems for fish.

What levels should I be aiming for?

Try to keep the nitrate level as low as possible. Some fish and inverts are particularly sensitive to raised levels. For inverts and sensitive marines, the levels must be below 15ppm. Tropical freshwater species should be kept below 40ppm, where possible.

How do I maintain a low nitrate level?

Overfeeding is a common cause of high nitrate levels. Regular partial water changes of 25% every week or two help to keep the nitrate level low. Do them with a syphon-powered gravel cleaner and remove decaying organic material from the gravel at the same time.

Nitrate-removing filter media will keep the nitrate level extra low, but isn't a substitute for plenty of water changes.

What effect will high nitrate levels have on my fish?

High levels of nitrate will stress your fish. This affects their immune systems and they are much more at risk from diseases such as whitespot.

Increased levels over a long term may reduce your fishes' lifespans, cause deformities and won't help much if you're trying to breed them - poor water quality doesn't put fish in the mood.

What should I do if the nitrate level in my tapwater is very high?

If you're keeping delicate fish or inverts any water you use for water changes should be run through a nitrate-removing resin, or a tapwater filter like a Nitragon, before use to remove the nitrate.

An RO unit is even better as it also filters out metals, pesticides, hardness etc, so it's ideal for softwater fish as well as delicate marine species.

The existing fish in my aquarium all seem healthy and my ammonia and nitrite levels are at nil. The tank has been set up for two years. Every time I buy new fish, they stop eating after a couple of days and then, usually within a week, they die. What's the problem?

You may have checked the ammonia and nitrite levels (which are very important), but what about your nitrate level?

If your nitrate has gradually risen over time since your tank was set up, your fish will probably have built up some resistance to it. But any new stock you add to a tank where there is a high nitrate level may fall ill or die.

Try to stay on top of tank maintenance and check how much you're feeding your fish. Remember to remove all uneaten food from the aquarium.