Why do fishes seem uncomfortable or die when tap water is used in the aquarium?

      This is due to the presence of chlorine in the water, which some fishes are very sensitive to. Customers can let tap water sit in a non-covered container for a few hours, to get rid of the chlorine, before using it in the aquarium. Alternatively, one can also utilize active carbon filters, or add a suitable amount of sodium thiosulphate (also called neutralizers), to shorten the amount of time required to remove the chlorine from tap water.