Should City water be treated before use with pet fish?
Yes. Your drinking water contains disinfectants to inhibit bacterial growth. These disinfectants can kill fish. Chlorine may be neutralized by adding appropriate chemicals that are available at most pet stores. Chlorine may also be removed with a granular activated carbon (GAC) water filter. Chlorine can be removed from water by letting a container of the water sit exposed to the atmosphere (uncovered) for at least 48 hours.
Some fish, particularly tropical fish and koi, are also sensitive to rapid changes in the temperature and pH of the water, even small changes. If you are going to change the water in an aquarium or pond, or add a significant amount of new water, consult an experienced fish care expert for tips on how to accomplish this without shocking or killing the fish.
Some people may be concerned that water harmful to fish might not be safe for humans to drink. This concern is unwarranted. Humans and fish use water in very different ways. When humans drink water, the chlorine or chloramines are neutralized by the digestive system before it enters our bloodstream. When fish "breathe" water, chlorine or chloramines enter their bloodstream directly. This interferes with the fish's ability to take oxygen from the water and can cause the fish to suffocate.
To further illustrate this point, consider orange juice. People are urged to drink orange juice for its health benefits. However, if a fish were placed in a bowl of orange juice, it would suffer a swift and unpleasant death.