The water that comes out of your tap has been treated to kill bacteria, and the thing that does that job kills fish at concentrations far below the level you can taste or smell. This page is about what is in it, what actually removes it, and one piece of standard advice that the water industry does not support.
Chlorine and chloramine enter a fish through the gills, straight into the blood. The US Environmental Protection Agency puts it plainly in its guidance on chloraminated water: "Chlorine and monochloramine can be harmful to fish because they directly enter their bloodstream through the gills."
The federal criteria for protecting freshwater life give the scale. Total residual chlorine should not exceed 19 micrograms per liter as a one-hour average, or 11 micrograms per liter over four days. Those are thousandths of a milligram per liter. Merck's veterinary reference gives adverse effects in fish "at chlorine concentrations as low as 0.02 mg/L". Drinking water is typically treated at levels far above that, because it is being made safe for people, not for fish.
Many water suppliers now use chloramine, which is chlorine combined with ammonia, because it lasts longer in the pipes. That is exactly why it is a bigger problem for a fish tank.
Pinellas County Utilities in Florida, a water supplier explaining its own treatment, states the difference: "Unlike chlorine, which evaporates when water sits for a few days, chloramines may not." And it is explicit about what that means for fish: "Chloramines must be removed from any water for fish tanks or ponds."
There is a second catch with chloramine. When it is chemically neutralised, the ammonia part is released into the water. In a tank with a working filter that is usually handled. In a brand new tank it is not.
Leaving tap water in a bucket overnight to let the chlorine gas off is repeated by the RSPCA and by OATA, the UK aquatic trade body. Both are serious organizations. But it only works for free chlorine, and water suppliers say chloramine does not leave that way. If your supply is chloraminated, and many are, a bucket of water that has stood for a day is still dangerous to fish.
You can find out which one you have. Water suppliers publish it, usually in the annual water quality report, and they will tell you if you call.
A dechlorinator does it chemically and immediately. The active ingredient in most of them is sodium thiosulfate. A University of Florida extension exercise comparing the methods gives the timings: sodium thiosulfate works immediately, while removing chlorine by aeration takes about 24 hours. Products sold for aquariums are usually formulated to deal with chloramine as well, and that is the label wording to look for if your supply is chloraminated.
Two things the Merck reference table is blunt about: the target for chlorine in an aquarium is zero, and you can buy a test for it. Most people never test for chlorine at all, which is odd given it is the one thing they add to the tank themselves every time they top it up.
Water evaporates; the things dissolved in it do not. Topping a tank up with fresh tap water replaces the water and leaves everything else behind, so the dissolved minerals slowly concentrate. It also adds untreated tap water straight into the tank, which is where a lot of unexplained fish deaths come from. Treat the water before it goes in, whether it is a top-up or a change.