Tap water must sit out and be put through aeration practices for a minimum of 12 hours before fish can be put into the water. The reason for this is that all tap water has chlorine in it and chlorine is harmful to fish.
By leaving the tap water out for this period of time with heavy water aeration, the chlorine evaporates, leaving the water free from chlorine. Many fish owners prefer to use dechlorinators instead. Most dechlorinators use sodium thiosulphate to remove the chlorine from the water. This substance is not poisonous to the fish unless it is over used, so it is necessary to follow recommendations and not to exceed those recommendations.