Can tap water be used to keep fish?

Tap water can be used for fish, but not directly. Tap water goes through settling, adsorption and disinfection, and chlorine-based disinfectants are usually added in that process, leaving a certain level of residual chlorine. Using it directly will harm fish health and can even kill them. So before using tap water, remove the excess chlorine; common methods are activated carbon or chemicals such as sodium thiosulfate. Because residual chlorine is volatile, you can also dechlorinate by leaving the water in sunlight, and only use it for fish once the chlorine is gone.