Dechlorinating Tap Water

Why municipal tap water is treated with chlorine or chloramine, how these differ, and how water conditioners neutralize them before water enters an aquarium.

Why tap water is disinfected

Municipal water suppliers add chlorine to kill disease-causing pathogens and prevent waterborne diseases such as cholera, dysentery, and typhoid. Chlorine works as an oxidizing agent, penetrating cell walls and reacting with intracellular enzymes and proteins. The same oxidizing action that disinfects water is harmful to fish at the gills.

Chlorine versus chloramine

Some suppliers use free chlorine, while others use chloramine, formed by combining chlorine with ammonia. Monochloramine is much less reactive and does not dissipate as rapidly as free chlorine, which makes it a more persistent residual disinfectant throughout distribution pipes.

Why removal matters

Because chloramine is stable, it cannot be reliably removed simply by letting water stand or aerate, unlike some free chlorine. Both compounds must be neutralized before water is added to an aquarium to protect fish and the nitrifying bacteria in the biological filter.

How conditioners work

Water conditioners commonly use sodium thiosulfate, which reacts with chlorine to form harmless chloride ions, and the reaction is effectively instant. When the source water contains chloramine, the conditioner reacts with the chlorine portion, breaking the chlorine-nitrogen bond and freeing the ammonia. Sodium thiosulfate is inexpensive, widely available, and a modest excess is not harmful to fish, which is why it is the basis of most dechlorinating products.

Handling released ammonia

Neutralizing chloramine leaves ammonia in the water. Where supply chloramine is in the typical 0.5 to 4 ppm range, roughly 0.2 to 1.3 ppm of ammonia can remain. A mature biological filter can convert this ammonia, but some conditioners are formulated to also detoxify ammonia for added safety.

Practical use

Dechlorinator is dosed to the volume of new water at every water change, before or as the water enters the tank. Aquarists on a chloramine supply should confirm their conditioner addresses both chlorine and the ammonia component rather than chlorine alone, because treating chloramine with a chlorine-only product still leaves ammonia behind. Local water companies publish whether they use chlorine or chloramine, which helps choose the right product and dose.