Can Nitrogen Kill Fish?

In aquariums, fish produce a number of nitrogen compounds as toxic biological wastes, which can harm fish if they build up in the aquarium. Understanding ammonia and other nitrogen compounds -- and how to control them -- is critical to keeping a happy, healthy aquarium. High levels of these nitrogen compounds can easily kill aquarium fish.

  1. The Nitrogen Cycle

    • In the home aquarium, a part of the nitrogen cycle makes it possible to keep aquarium fish alive. Ammonia enters the aquarium as fish waste. A set of bacteria break down the ammonia into less toxic nitrite. From here, a second set of bacteria break down the nitrite into nitrate, which in small amounts does not harm fish. Aquarium plants, live rock and water changes remove nitrate from the aquarium. In the wild, the nitrogen cycle has several more steps, forming a complete cycle whereby nitrate breaks down into atmospheric nitrogen or gets absorbed by plants and converted back into food. However, in aquariums, the full nitrogen cycle cannot run, but this segment of it makes it possible to keep fish alive in aquariums. While aquarium plants do absorb some nitrate, and trace amounts break down into to atmospheric nitrogen, aquariums cannot rely on this process. This nitrate buildup is a big reason why you have to perform regular water changes on fish tanks.

    Other Sources of Ammonia

    • In aquariums, fish waste is the main source of ammonia but not the only one. Anytime something biological decays, ammonia is produced. Uneaten fish food, dead animals and dead plants all produce ammonia. For this reason, you need to take care to avoid overfeeding fish and remove any dead organisms as quickly as possible.

    Effects of Ammonia and Nitrite

    • Ammonia and nitrite both produce toxic effects. These nitrogen compounds are classified as tissue irritants. This means they harm tissue directly through corrosive action. Ammonia and nitrite first burn the skin and gills of fish. Since fish excrete ammonia though their gills, damage to the gills causes even more ammonia to build up in their tissues, causing further damage. Fish suffering from ammonia and nitrite poisoning will typically flap their gill covers more rapidly than usual and "gasp" at the surface of the aquarium. Ammonia and nitrite can easily kill aquarium fish.

    Cycling an Aquarium

    • The process of establishing the bacteria that control ammonia and other nitrogen compounds in the aquarium is called cycling the tank. In this process, you add a few hardy aquarium fish and monitor the ammonia and nitrite levels daily. First, ammonia will spike as the fish produce it. Then, the bacteria that metabolize ammonia will multiply enough to meet the demand, and ammonia levels will fall. As ammonia falls, nitrite levels will rise, since the bacteria that break down ammonia turn it into nitrite. After this, the bacteria that break down nitrite will multiple, decreasing the levels of nitrite. Once ammonia and nitrite reach undetectable levels, you can perform a 25 percent water change and add a few more fish. You will repeat this process until the aquarium is fully stocked.