Tap water contains Chlorine or Chloramines, which are harmful to fish. Use a good quality water conditioner / water ager to remove these harmful chemicals from the tap water before adding it to the fish tank. Always use a water conditioner when adding new water to the pond, freshwater or saltwater aquarium.
We carry are great selection of quality water treatments from brands like Seachem, Sera, API, Nutrafin and Fluval to help make your tap water safe for your fish.