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 membranes. Interpet Fresh Start rapidly remove these toxic chemicals and metals and contains a natural colloid which coats the fish with a protective barrier. When to use Interpet Fresh Start: When starting a new aquarium or carrying out regular water changes Whenever you top up your aquarium with tap water If you suspect heavy metal poisoning One 100ml bottle treats 500 Litres