Municipal water in the USA has to be treated with chlorine. Federal law requires ammonia to be injected into the water system too, making chloramines. Chlorine can be removed easily be using a dechlorinator. (works in seconds if used in the right proportion) Unlike chlorine, chloramines CANNOT be easy removed, only neutralized.( I use Prime) In order for it to go away naturally the source water needs to be violently agitated for several days. Low levels of chloramines can kill sometimes even several days after a water change. Day 1 water change, day 4 or 5 fish die, why? Probably related to the water change, usually something stupid OR low levels of chloramines.
It's easy to test. Go to a pool supply. Buy a pack of dpd 1 and dpd 3. (about a buck a pack) fill a small vial (test tube) with water, drop in a tablet of dpd 1. If it turns pink, you've got chlorine. Hold the tube in front of a WHITE card or shirt. Now, drop in a tablet of dpd 3. If the water changes to a richer pink or even a wine color, the difference is the chloramines. This second part should take several minutes... 20?
If the water smells like chlorine the ammonia injector is probably broken. If it smells like ammonia, the chlorine isn't there, the guy with the shovel called in instead of showing up for work. No rain... too many chemicals. Lots of rain... more work with the shovel to add chlorine.
Unless you've got your own water source AND it's been tested, spend a few cents per gallon and treat all of the new water you add to an existing tank. A pint normally does around 240 gallons.
If some one hands you a glass of something, do you ask what it is, or just drink it down?
No one is getting rich selling you dechlorinator. There are more profitable and creative ways to steal.