Signs of Nitrite Toxicity in Aquarium

Algae in a tank may indicate nitrates are high.
Algae in a tank may indicate nitrates are high. (Image: Jupiterimages/ Images)

Different species of aquatic creatures in both freshwater and saltwater aquariums have varying levels of tolerance for nitrates. Fish-only aquariums, for instance, may be able to host higher nitrate levels than tanks with corals or shrimp. Regardless, it is important to know what's a proper level and to be able to recognize the signs of high nitrates and work quickly to correct the matter for the health of every living thing in the captive habitat.


Nitrates, or NO3 compounds, are composed of nitrogen and hydrogen atoms. Decaying food, plants and fish waste form ammonia, which creates nitrates at the end of the nitrogen cycle. A large amount of nitrates in a tank creates nitric acid.

Testing and Levels

Test to determine levels of ammonia and nitrates weekly, using an over-the-counter test kit. Even a small amount of ammonia or nitrite in a tank can cause fish harm. However, a minimal level of nitrates in a tank is generally acceptable. When testing your tank, the ammonia and nitrite should be at 0 ppm, whereas the nitrate may be up to 40 ppm and not cause harm.

Signs of High Nitrates

High levels of nitrates promote algae growth in a tank. This generally causes the water to appear cloudy or dingy. When fish experience a rise in nitrate levels, they become sluggish and may show no signs of movement at all. Corals, on the other hand, may experience stunted growth, according to the Advanced Aquarist website.

Reducing and Preventing Nitrate Toxicity

A carbon filter that removes ammonia and promotes healthy bacteria is the first line of defense against high nitrates. When nitrate toxicity occurs, immediately do a 50 percent water change and clean the rocks to remove waste. Keep only a few fish at a time to avoid overstocking, and feed the fish only small amounts of food to prevent decay. Look into your species' feeding protocols: You'll let some fish feed for only a minute before you remove leftover food, while other species feed for much longer.

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