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.