How to Remove NO2 From Your Aquarium

Fish waste and excess food in a new aquarium can build up to create dangerous toxins that may kill your fish. Ammonia, nitrite (NO2) and nitrate occur naturally as part of the nitrogen cycle about three weeks into a new set-up. The toxins can only be detected with a water testing kit, and your tank will look clean and healthy even when ammonia, nitrite and nitrate levels are dangerously high. Prevention is best, but if NO2 develops, partial water changes are the most effective method of treating the problem.

Follow the manufacturer's instructions on the test kits and check your aquarium water for ammonia, nitrite and nitrate. The nitrogen cycle produces ammonia first, which dissipates and turns into nitrite. Nitrite levels peak before dropping off and forming nitrate. A new tank set-up is very likely to experience all three toxins in the first three months, and if left unchecked, fish will die. Water test kits provide an easy to follow colour chart to enable you to see if you should take immediate action.

Reduce nitrite quickly by performing a 50 per cent water change if the levels are reading high on your test kit chart.

Add plants to your aquarium. Live plants feed on nitrite and nitrate, helping to stabilise your tank by playing an important part in the nitrogen cycle.

Replace a third of the water every week for the next two months. You should see the levels drop until the ammonia disappears completely. Nitrite levels often spike again at this point, but further partial water changes will make the water stable.

Add a beneficial bacteria solution to enhance the biological properties of an aquarium filter. The microbes speed up the nitrogen cycle and create an environment that reduces toxins naturally. Use the solution when adding fish to your aquarium or carrying out a water change.


Overfeeding is a common cause of excessive nitrite in a tank. Feed your fish less or not at all when nitrite is high.


Do not change too much water at a time because it causes other environmental instabilities. If you clean the whole tank and replace all of the water, you destroy good bacteria and start the nitrogen cycle again.

Things You'll Need

  • Ammonia test kit
  • Nitrite test kit
  • Nitrate test kit
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About the Author

Alex Burgess has been a professional writer since 1990, specializing in travel, herpetology, lifestyle, fashion, health and fitness. Her work has appeared in various British newspapers, magazines and international online publications. Burgess studied design before working as a journalist in England.