Snails reproduce in a variety of ways. Most often it's either fertilized eggs or broadcast spawning. If they have laid eggs, they were already fertilized. As far as I know, some of the egg laying snails in our aquariums are able to store sperm, so even one snail in a tank can have fertilized eggs. It's not often to have snail larvae grow up into even sub-adults. They usually get eaten or starve to death because of lack of food. There's not much you can do about it really.