Water from the tap contains chlorine. This kills the bacteria in the water making it safe for human consumption. However the chlorine in tap water is also poisonous to fish. Therefore it needs to be removed from the water before fish can be placed in it.
To treat tap water you need to add a dechlorinator or water conditioner, such as tap safe, to make the water safe.
This should be added whenever you set up a new tank and also at every weekly waterchange.
Click images for more info.
