Okay, your mollies and platys are both live bearing fish. They breed very easily and if you have females, they probably were pregnant when you got them from the store. How many of each do you have? To tell if they are female, look at their small bottom fin, just behind their belly. If is is triangular, it is a female. If it is a straight line and points toward their tail, it is a male. If the females are pregnant, they will have a dark spot just above and in front of that fin. Check them out and let me know what you find.