Tap water in your fish tank? Here's what chlorine does to their gills

A fish does not drown in chlorinated water. It suffocates in it, slowly, while looking perfectly fine until it isn't. The gill is the organ doing the work, and chlorine is exceptionally good at destroying it.

Municipal water treatment adds chlorine, and in most Indian cities now, chloramine, a chlorine-ammonia compound, to kill bacteria and make water safe to drink. For humans, the concentrations used are harmless. For fish, the same concentrations are corrosive at the tissue level. The difference is exposure route: you drink water and your gut handles trace disinfectants. A fish pulls water continuously across gill membranes to extract dissolved oxygen. Every litre that passes over the gills is a litre of chlorinated water in direct contact with living tissue.

Fish gills are built for gas exchange: oxygen in, carbon dioxide out. To do this at any useful rate, the gill lamellae, the fine, leaf-like folds that line each gill arch, are thin-walled, densely supplied with capillaries, and covered in a mucus layer that keeps them moist and protected. This architecture is also what makes them vulnerable. Chlorine is an oxidising agent. When it contacts the gill's mucus layer, it oxidises and strips it away. Without that layer, the epithelial cells underneath are exposed. Chlorine then attacks those cells directly, causing inflammation, cell death, and a thickening response as the tissue tries to protect itself. The thickened, damaged lamellae cannot exchange gas efficiently. The fish experiences the equivalent of breathing through a wet cloth that keeps getting thicker. Oxygen uptake falls. The fish shows laboured breathing, surface gulping, and lethargy, signs that are easy to misread as a water quality problem in general, rather than the specific cause.

Plain chlorine dissipates. Leave tap water in an open bucket for 24 hours and much of the free chlorine off-gasses. Chloramine does not. It is a more stable compound, added precisely because it persists through long pipe runs and storage tanks. Most municipal water supplies in Indian metros now use chloramine rather than chlorine alone, which means the old advice, let the water sit overnight, no longer works. Chloramine also penetrates gill tissue more deeply than free chlorine and is harder to neutralise with basic dechlorinators. A product that handles free chlorine only will leave chloramine intact. The label matters: look for a conditioner that explicitly states it neutralises chloramine, not just chlorine.

The fix is not complicated, but it has to happen before the water touches the tank. A sodium thiosulphate-based dechlorinator neutralises free chlorine almost instantly. A conditioner that also contains a reducing agent for chloramine, most modern liquid conditioners marketed for aquarium use do, handles both. Dose the conditioner into the bucket before adding tap water, not after and not directly into the tank during a water change. Temperature matters too: cold water holds dissolved gases differently and stresses tropical fish; match the new water to the tank temperature within a degree or two before adding it. For tanks with sensitive species, discus, wild-caught fish, or any fish already showing gill stress, a dechlorinator that also binds heavy metals and coats gill tissue with a slime substitute adds a meaningful margin. If a fish is already surface-gulping after a water change, increase surface agitation immediately to raise dissolved oxygen, and do not add more untreated water. A fish showing gill damage, rapid operculum movement, pale or visibly inflamed gills when you look closely, loss of colour and appetite persisting more than 24 hours after a water change, needs a vet with fish medicine experience, not a wait-and-see approach.