How to get nitrates down in an aquarium

Nitrates are the bane of many an established fishkeeper. But there are some quick and easy fixes. 

What is it?

Nitrate is part of the Nitrogen cycle and is formed when ammonia is converted by bacteria. Nitrosomonas bacteria convert ammonia into nitrite, before Nitrospira convert nitrite (NO2) to nitrate (NO3). Nitrite and nitrate sound very similar and are often confused, but nitrite is very toxic to fish even in low levels, whereas nitrate is less toxic and builds up.

A nitrite level of just 1 part per million is enough to poison and kill most aquarium fish, yet nitrate levels can be 100 times that in some cases. 

High nitrates are linked by some to nuisance algae, and Old Tank Syndrome, where nitrates rise, pH drops, and any newly introduced fish are shocked and die, despite ammonia and nitrite tests proving negative. 

Nitrate can also be present in tapwater, so you may introduce nitrate unknowingly, when replacing old tank water with new tap water.


The quickest and easiest way to get nitrates down is to change the water. As long as your tapwater has a lower nitrate level than your tank water, by replacing it, your nitrate level will go down. Test the nitrate level in both your tank and your tap, to ensure that this is the case. 

As long as the water you are replacing is of the right temperature, and has been treated for chlorine or chloramine, you can conduct water changes of 50% without issues. 

If the nitrate level in your tapwater is high however, at 40ppm or more, most people opt not to continue using it, and turn to RO water instead. RO stands for Reverse Osmosis, and the process removes nitrates, phosphates, chlorine and minerals from tapwater.

Plumb one at home and you will have an unlimited source of purified water to carry out water changes. RO water is soft too, benefiting fish which prefer water with a low pH.


Live aquatic plants are nature’s nitrate filters, and readily soak up nitrates, and use them as fertiliser. Heavily plant any nitrate-laden aquarium with fast growing live plants, and they will remove all the nitrate within days or weeks. 

Aquascapers actually add nitrate in the form of Nitrogen, or N, as plants can actually become deficient of this key nutrient after they’ve used it all up. Live aquatic plants have many other benefits too including providing shelter for small fish and fry, providing places for fish to scatter eggs, and they help to fight off algae. Just ensure that any planted aquarium has the right lighting, and other fertilisers, to keep the plants healthy and growing. 


Nitrate can be lowered by the addition of Tetra Nitrate Minus. Use as directed, add to established tanks, and you should see the nitrate curve lessen over time when compared to not using it. Along with Tetra Easy Balance, this is useful for standard community aquariums where regular maintenance cannot always be carried out as often as it should. 


Nitrate can also be reduced by special filter media like Seachem De Nitrate. Bacteria can be both aerobic (oxygen consuming) and anaerobic, and it’s anaerobic bacteria which are good at reducing nitrate. Although not a rapid fix, once the anaerobic population of bacteria are established within the media, nitrate should reduce. 

Nitrate sponges

Some Nitrate sponges work in a similar way to other nitrate reducing media, encouraging bacteria to reduce nitrate from within, while others work chemically, absorbing nitrate and locking it away. Both can be effective, and the mainstay of many leading filter media systems including Juwel and JBL.

Feed less, stock less

The nitrate level of any aquarium should be in direct relation to the amount of ammonia being produced by the fish. The more fish you have, the more nitrate will be produced. So tanks with chronic nitrate problems are usually overstocked, and over fed. Keep fewer, smaller fish, and don’t overfeed, and less ammonia and subsequent nitrate will be produced.   

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Jeremy Gay is an author and freelance aquatic specialist. A former editor of Practical Fishkeeping magazine, he offers a wealth of experience on all things aquarium and pond.

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