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Aquarium Advice

Aquarium Maintenance: How to Keep your Nitrate and Phosphate Levels Down

Are you battling with high levels of nitrate and phosphate in your aquarium? If you’re having issues with build-ups, or if you’re not sure how to prevent them from escalating to begin with, you’re in the right place.

In this blog we will share our top tips for controlling nitrate and phosphate in your tank.

What are nitrates and phosphates and where do they come from?

Nitrates (NO3) are produced when the bacteria in your filter breaks down ammonia, first converting it into nitrite before eventually converting it to nitrate. Ammonia and nitrite are highly toxic to your fish, whereas nitrates are less so. However, if allowed to build up too much, nitrates will eventually prove harmful to your aquarium life.

Phosphates (PO4) are also not directly harmful to your fish, even at high levels, but they can indirectly wreak havoc in your tank. Phosphates are the natural product of waste breakdown, including uneaten food, decaying plants and fish faeces. If allowed to build up, phosphates can contribute to algae blooms within your tank, which in turn can starve the water of oxygen and harm your fish.

Issues with aquarium nitrates and phosphates are common and can easily be managed with a few simple measures. First, though, you need to find out what you’re dealing with:

Test your aquarium water

Before you can begin tackling nitrates and phosphates, you need to know how much is currently in your aquarium water. This will help you figure out how much action you need to take and how serious your build-up is.

You can use nitrate and phosphate aquarium test kits to determine the levels in your tank. Some kits use colour dyes or strips to indicate levels in your tank, and some give readings in milligrams per litre (mg/l) – nitrates are harmful when over 50mg/l, and phosphates should never be allowed to exceed 1mg/l.

Reducing and preventing nitrate and phosphate build-up

Once you know the levels of PO4 and NO3 in your tank, you can take the following steps to reduce or prevent build-ups:

Water changes

Sudden and drastic changes to your water chemistry can be just as harmful to your fish as the substances you’re trying to reduce, so you need to keep any changes gradual. If you’ve detected very high levels of phosphates or nitrates in your water, start by doing a 25% water change, which you can do with a jug, siphon or gravel vac. Repeat the process daily and keep testing the water until you’ve achieved a safe level.

If your nitrate and phosphate levels are acceptable, you should still maintain a regular water changing schedule, changing 15-20% of the water once a week. This will help keep your water free of many common harmful substances.


Overfeeding your fish leads to a build-up of uneaten food, which decays and produces phosphates and other nutrients. This in turn can lead to harmful algae blooms and green water. Keep a close eye on your fish when feeding and take note of how much they eat, making sure to only feed the amount they will eat within a 2-3 minutes; this will allow you to determine how much they actually need, helping you reduce waste.


Fish produce their own waste, so it makes sense that having more fish in an aquarium generates higher levels of faeces, which leads to higher NO3 and PO4 levels. The general rule of thumb is to allow one gallon of water for every one inch of fish, though this is only a rough guide. If you think you might have too many fish in your aquarium, consider upgrading to a bigger tank to accommodate them and their waste levels.

Nitrate and phosphate absorbing filter media

Nitrate and phosphate-absorbing filter media can be added to practically any aquarium filter to help reduce levels in the water. Try to avoid carbon media if possible, as this can sometimes leach phosphates into the water, though some are specifically formulated not to. Make sure you carefully read the accompanying information and instructions before trying a new filter media.

You also need to ensure your filter media is regularly changed, otherwise the substances you’re trying to remove will simply be returned to the water when the media is used up, and your efforts will have been wasted.

Keep live plants or live rock

Planted tanks are the most natural way to keep nitrate levels low, as plants utilise a great deal of the ammonia and nitrites in the water. Live rock and sand also contain microorganisms that feed on the nitrates, preventing them from building up. Do bear in mind, however, that heavily planted tanks perform best when only lightly stocked with fish.

Additives & treatments

If none of the above seem to be making a big enough difference to the high levels of NO3 and PO4 in your aquarium, you can try adding chemical additives and water treatments, which are specially formulated to tackle these substances. Many of the top aquatic brands like Seachem and Tetra offer additives that tackle algae, detoxify ammonia and nitrites and remove phosphates and nitrates from the water. Ideally, however, chemical treatments should be used as a last resort, and not used as your go-to method for tackling these substances in the long term.

If you’re struggling with nitrate and phosphate levels in your aquarium and want to know more about the safe use of additives and other preventative measures, give our team of experts a call on 01642 612419.