chlorine poisoning in fish

Symptoms of chlorine poisoning in fish


At many pumping and water supply stations, chlorine is added to drinking water as a disinfectant. Chlorine can be added to water in various forms: either as elemental chlorine gas, as chlorine dioxide, or as hypochlorous acid (an aqueous solution of chlorine gas). The disinfecting effect is based on a series of reactions of chlorine or chlorine dioxide readily soluble in it. Two different chlorine compounds are formed: hydrochloric acid (HCl) with a low oxidation state of chlorine and hypochlorous acid (HCl) with a high oxidation state of chlorine.

Hypochlorous acid decomposes quite slowly in our water supply network since sunlight does not penetrate there. In addition, in order to kill bacteria, so much chlorine is introduced into tap water that it reaches our taps even in its free form. Thus, free chlorine can get into our aquariums with drinking water from a tap and decompose into hydrochloric acid with the formation of chloric acid. Strongly bleaching and oxidizing hypochlorous acid acts on the gills and scales of fish, causing great damage.


Fish in chlorine-containing water show signs of severe poisoning, difficulty breathing, they are forced to rise to the surface, become shy, change color. They begin to rush around the aquarium in search of clean chlorine-free water. In cases of severe poisoning, the fish soon become slow and then die.


Due to the strong oxidizing effect of chlorine-containing water, sensitive gills and blood are damaged in the first place. In addition, hyperplasia of the gill epithelium occurs and disturbances in the general metabolism occur. In poisoned fish, the gills become faded, almost discolored, skin lesions appear.

Chlorine is very toxic to fish. At a chlorine concentration of 0.016 mg / L, numerous injuries appear, and even at a concentration of 0.001 mg / L, the behavior of the fish changes. Drinking water may contain chlorine in a concentration of up to 3 mg / l, and this is absolutely not suitable for fish.

Research methods

If the above symptoms appear, check the water for chlorine. It is recommended that you buy quick test kits to determine the chlorine concentration of 0.001 mg / L.


In case of severe, acute chlorine poisoning, you can try to save the fish by adding sodium thiosulfate, which is able to bind chlorine, to the aquarium water.


As a precautionary measure, I recommend that you first find out about the real concentration of chlorine in your tap water, for which you need to call to the according service, a water station, or your local utilities. Even minor amounts of chlorine should be neutralized either by adding sodium thiosulfate to the water or by passing water through a home filter. The method of neutralization with sodium thiosulfate has the disadvantage that you do not know how much it needs to be added to water to neutralize chlorine, and secondly, the reaction products will remain in the water, even if they are relatively harmless. The cleaning method through a home filter is the simplest and most effective. To do this, you only need a large container (a polymer barrel, another aquarium, etc.) in which you pour tap water, passing it through a good, effective filter, and leave it for several days in the light. Free chlorine is converted to harmless calcium chloride, which occurs in certain sizes due to the presence of calcium in tap water. Chlorine is completely removed by heating tap water (or thawing after freezing).


Activated carbon can also remove free chlorine from water. At the same time, the activated carbon absorbs not only free chlorine but also accelerates the process of conversion of chlorine into hydrochloric acid through the decomposition of hypochlorous acid. This method works optimally only under a certain pressure. For these purposes, you need to have a stationary tank with activated carbon, directly connected to a water tap.