What causes high nitrites in the aquarium?

While you may have heard that ammonia is deadly to aquarium fish, you may not have heard that nitrites can be just as dangerous. It is important to understand what causes nitrites in the aquarium and how they can be quickly removed if they enter your saltwater or freshwater system as they can result in dead fish and invertebrates.

Read on to find out everything you need to know about the nitrogen cycle, nitrites and how to remove them from your aquarium water!

What causes the nitrites in the aquarium water to be high?

Nitrites (NO2) are the result of ammonia being processed by nitrifying bacteria as a kind of biological filter. These nutrients are usually a sign of a bike tank, overfeeding, overcrowding, or other poor maintenance of the fish aquarium.

Bicycle tank. In order to make an aquarium habitable for fish and other aquatic life, it must first go through the nitrogen cycle. This includes a source of ammonia that beneficial bacteria first convert to nitrite and then to nitrate. Over the course of a month, the ammonia level rises, which leads to a detectable level of nitrite and a decrease in traces of ammonia. The nitrite levels then decrease while the nitrate levels increase. If only nitrate levels are present, the aquarium is considered fully cyclical and a water change is strongly recommended.

Most of the water quality problems associated with a newly established aquarium can be traced back to “New Tank Syndrome,” which occurs when the bacteria are not fully established enough to properly meet the nutritional needs of the freshwater or saltwater aquarium system balance.

Overfeeding. Overfeeding can lead to a lot of excess nutrients in the water column, which can lead to algae and overall poor water quality. If leftover food is not eaten, it can lead to rot and waste, causing ammonia spikes. This ammonia is not only immediately toxic to fish and invertebrates, but the nitrogen cycle also leads to dangerous nitrite and nitrate levels.

Overcrowding. While you may not overfeed your fish, overcrowding can also lead to increased amounts of waste in the water column. Many fish and invertebrates excrete body waste with ammonia, which is converted into nitrite by the nitrogen cycle. More fish means more ammonia, which corresponds to more nitrites and nitrates in the aquarium.

Bad aquarium maintenance. Even if you let your new tank drive all the way up and take care not to overfeed or overfill with fish, there is still a chance you are not giving your aquarium the attention it needs to thrive. In general, good aquarium maintenance will require partial water changes every 2 to 4 weeks and flushed / new filter media approximately every month. Dirty substrate or clogged equipment can leak excess nutrients into the aquarium water if not controlled, even in an established tank.

How do you fix high nitrites in aquarium water?

Fortunately, avoiding nitrites in your aquarium water is pretty easy. First of all, you will need a saltwater or freshwater test kit that will show you the exact water parameters in your saltwater or freshwater tank. Usually nitrites are not the only ones to be present when there is an obvious problem, and other levels likely need to be balanced as well.

Once the problem is identified, it is usually recommended to do a partial water change every day until the nitrite reads 0 ppm with an accurate test kit. This will also help reduce traces of ammonia and keep nitrates down. During these water changes, it is also recommended to vacuum the substrate to remove any stubborn debris. However, if the substrate is perturbed too much all at once, the water parameters can fluctuate, resulting in a mini-cycling process.

Changing the water and vacuuming the substrate are usually the easiest and most immediate way to remove ammonia and nitrite from the aquarium. Supplementing filter media and chemical solutions is usually not recommended as it is only a temporary solution. In newer aquarium configurations, however, nitrifying bacterial additives can be added to establish and strengthen the bacterial populations and to process the high nitrite levels more quickly.

Also, make sure your tap water does not contain nitrites by checking it regularly with a test kit. If your tap water has lots of extra nutrients in it, it may make sense to switch to distilled or reverse osmosis water.

Do High Nitrites Kill Your Fish?

Yes, high nitrites in aquarium setups kill your fish and invertebrates. Due to its chemical structure, nitrite can bind to hemoglobin, which otherwise oxygen in the blood should bind to. Poor water chemistry in your aquarium can cause your fish to suffer from nitrite poisoning, in which they suffocate due to a lack of oxygen in the blood.

How long can fish with high nitrites live in the aquarium?

If both ammonia and nitrite levels are present, aquarium fish can die within hours of adding them to the aquarium. With only high nitrite levels, fish may survive for a few days but are in extreme pain. Some signs of nitrite poisoning include lethargy, loss of appetite, redness around the gills, difficult breathing, and gasping for air on the surface of the water.

If you see any of your aquarium fish showing these signs, test the water parameters and take the right steps to correct the water chemistry as soon as possible.


Nitrite is just as dangerous as ammonia for many fish species and invertebrates. Nitrites are often the result of newly set up aquariums in which the water quality fluctuates from day to day, but can also be the result of overfeeding, overcrowding or poor aquarium maintenance.

Always make sure you have the appropriate test kits on hand so that you can find out the problem as soon as possible and give your aquarium fish and invertebrates the best chance of survival. Also, make sure that you never bring livestock into an aquarium that is being bicycled, even if you are treating them with bacterial supplements.

If you have any questions about the nitrogen cycle, partial water changes, or how to combat high nitrites in your own saltwater or freshwater aquarium, please leave a comment below!


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