I dont know about your deraileur but atleast Shimanos deraileurs are adjustable in the way that you dismantle it, open the part where the spring is (where it is attached to the frame) and voila, the end of the spring sit in a hole, and there is another hole next to it, just move the end of the spring to that hole and you got more tension.
But please, get my word verified before you start taking things apart. And as I said, this is for Shimano rear deraileurs.