Chlorine and Chloramine in Tap Water: Why They Kill Your Fish
Tap water is treated to be safe for humans to drink, not to be poured straight into an aquarium. Chlorine, and increasingly chloramine (a more stable chlorine-ammonia compound used by many water utilities), disinfect water by killing microorganisms. The problem is that fish gills are thin, heavily vascularized tissue, just as vulnerable to these disinfectants as the bacteria they target. Chlorine ecotoxicology in fish is well documented (research on Google Scholar): even at concentrations considered safe for drinking water, it causes gill damage and respiratory distress and can be fatal within hours. Our maintenance benchmarks are in the fish care and tank hygiene section.
Chlorine vs. chloramine: an important difference
Free chlorine evaporates fairly quickly if water is left exposed to air for a few days, aerated or slightly warmed. Chloramine, by contrast, is specifically designed to stay stable longer in pipes: it will not disappear on its own in the open air within a reasonable time. Relying on the old "let it sit overnight" habit is therefore risky if your utility uses chloramine, which you can easily check with your local water supplier.
A water conditioner, every single time
A commercial water conditioner neutralizes chlorine within minutes and, for suitable products, also breaks the chlorine-ammonia bond in chloramine. This is a step to take at every water change, not only when setting up a new tank: we cover product choice in our guide to reverse osmosis water for aquariums. Under-dosing leaves fish exposed; over-dosing adds nothing: follow the dose stated for the water you're adding, not for the whole tank.
A risk to your biofilter too
Chlorine doesn't only target fish: it also kills the nitrifying bacteria in your filter that convert toxic ammonia into nitrite and then nitrate. Untreated water poured straight into the filter can trigger an ammonia spike days later, even if no fish showed visible harm at the time. The nitrogen cycle is explained in our article on water tests and the nitrogen cycle.
Frequently asked questions
Can you just let tap water sit out instead of using a conditioner?
Only if your utility uses plain chlorine and you leave it several days in a wide, aerated container. With chloramine this isn't reliable: a conditioner remains the safest option.
Does chlorinated tap water always smell strong?
No, smell is not a reliable indicator of the dose present or of the treatment method your utility uses.
Do you need to condition water for a small water change too?
Yes, every time, regardless of the volume added, to protect both fish and filter bacteria.
This guide is part of Planète Pets’s Fishkeeping universe. Our advice is general in nature: for any health concern, your veterinarian remains the only reference.