Koivet.com | Fish Health | Dr. Erik Johnson

Chlorine (You better know about this). When water comes out of your hose, in most areas, it’s “city water”. This means that it is accumulated by and “made drinkable” by the city. Then the water is supplied to your house. They almost invariably “chlorinate” the water. This means they add chlorine (and sometimes chloramines) to the water. These compounds are supposed to make the water safe for you to drink and use. Chlorine and Chloramine are toxic to fish.
So; every time you add new water from the hose or the tap, you should apply some “dechlorinator” to the volumne of water you’re adding. Dechlorinators are inexpensive and effective. It is not possible to use ‘too much’ dechlorinator because this compound, (Sodium thiosulfate) is not toxic to fish.
Experienced advice: Always treat incoming city water with a sodium thiosulfate based dechlorinator. Never skimp. You’ll hear the pundits say that you can mist or spray the water through the air into the pond and that will neutralize the Chlorine. This is true to a very limited extent and does not consider that municipalities will add variable amounts of Chlorine and Chloramine to the water depending upon how the water checks out that day. Again, to compensate this variable; always dechlorinate.
SHORT AND SWEET: Dechlorinate every time you do a water change. Dechlorinator is cheap and it’s easy and safe to use.