Interpet fresh start tap water conditioner becuase tap water contains chlroine and heavy metals it is toxic and aggressive to fishes delicate skin and gill membranesr interpet fresh start rapidly remove these toxic chemicals and metals and contains a natural colloid which coats the fish with a protective barrierr when to use interpet fresh startr r when starting a new aquarium or carrying out regular water changesr whenever you top up your aquarium with tap water if you suspect heavy metal