Distilled or tap water in freshwater aquarium?

In my 10 g platy aquarium I've been using distilled water from the water cooler, is this wrong? Should I be using just regular tap water? I've got a stress coat (tap water condioner, and a bottle of prime that removes chlorine, and also have some freshwater aquarium salt), should I be using some of this stuff every time toping up the aquarium with water, and is to much bad? Fish are being real inactive, just kinda lying on the bottom of the tank, what can I do to make them a little more active? 2 male and 2 female platy (1 pregnant other gave birth to 11 aborted fry a week ago). When males are done impregnating the females do they just ignore the ladies? I know I need another female, was actually thinking of a swordtail, is this a good idea?