If you don't want an exploding population, just let the fry be, and they provide food for your other fish. Unfortunatley, this is the way it usually goes once you raise your first batch of fry. lol Mollies, like any other livebearer (correct me if I'm wrong please) can hold up to 6 "packets" of sperm, and decide the optimum time to 'become pregnant'. In a well cycled tank, is usually always. They breed worse than rabbits, and without a bit of population control, things can get out of hand unless you breed those things in the thousands in a pond for any kind of profit.