Depends on the fish and condition of the tank.I do 50% without dechlor on all my tanks weekly.
Although, it does depend on how much chlorine your water supply has, I'm betting ours has little, while I've heard of some people water actually smelling of chlorine, in which case dechlor would obviously need to be used.
Depends where you live and the Cl concentration in the tapwater. Some places are low and you don't need dechlor. However in parts (certainly around where I am), the smell of the chlorine (and chloramine I bet) is very strong and will definitely kill filter bacteria.I do 50% without dechlor on all my tanks weekly.
No worriesOops, sorry mate. Should have read on a bit more