dechlorinator, you can buy any number of water treatments which remove chlorine, you just add them to the new water you put into the tank or you add them directly to the tank when doing a water change.
there are some preferred brands but i've never found much difference between them, just make sure you get something that treats chlorine, chloramine and heavy metals. if you have a large tank then buying pond dechlor often works out more cost effective as they are more concentrated, it's exactly the same stuff. not always worth while on small tanks as it's hard to dose because of the scale and while you can safely double dose doing more than that is not recommended as there is some evidence to show it can interfere with your cycle.