How Can Chlorine & Chloramine Affect Fish in an Aquarium?

Aquarium fish are highly sensitive to chlorine and chloramines in their water. Chlorine is dangerous, even in concentrations of less than 0.5 parts per million (ppm), as it burns the gills and can cause the death of aquarium fish. Chloramine is a mixture of chlorine and ammonia and does not dissipate out of water, as chlorine does. It is not typically removed by commercial aquarium chlorine neutralizers and can be lethal to aquarium fish. Tap water contains both of these products.

  1. Chlorine Toxicity

    • Chlorine burns the edges of gills and destroys cells. This seriously interferes with gaseous exchange and the fish is no longer able to use dissolved oxygen in the water. Affected fish exhibit labored breathing and may concentrate at the water surface. Chlorine dissipates out of water over time, particularly if the water is vigorously aerated. It is therefore possible to remove chlorine from tap water by leaving the water in a bucket with an air stone for three or four days

    Chloramine Toxicity

    • Chloramine does not destroy the gill epithelium as chlorine does, but enters the bloodstream, where it binds to iron in the hemoglobin of red blood cells. This action interferes with the cell's ability to bind oxygen. Chloramine toxicity is affected by the temperature and pH. The toxicity is also influenced by the presence of chlorine and the age of the fish, youngsters being more susceptible. Chloramine is not easily dissipated from water, even in the presence of very heavy aeration. Chloramine can be removed with the use of activated carbon or by using a commercial aquarium product specially designed for treating water containing chloramine.

    Ammonia Toxicity

    • The ammonia in chloramines is highly toxic to fish, particularly to youngsters and fry. Ammonia toxicity is elevated in water with a high pH, which puts African cichlids and marine fish, at greater danger. Exposure to ammonia is similar to nitrite (NO2-) toxicity, in which the fish become lethargic and either hover near the water surface, or remain on the aquarium floor. Fish can die within 24 hours when exposed to ammonia.

    Destruction of Bacteria

    • Chlorine is not only directly dangerous to fish, but causes indirect problems by destroying the beneficial bacteria that filter aquarium water. These bacteria break down the toxic metabolic waste products that are produced by fish. Ammonia, nitrite (NO2-) and nitrite (NO3-) are all converted by these bacteria colonies and it will not be possible to filter the aquarium water biologically in their absence.