Snails can also become a pest too, if you have one i dont think you will have much problem but if there are two, then you ought to be careful because livebearers are class water bunnies, these snails will put that name to shame, because they will breed faster than bacteria, and so infest your tank and make it a unpretty sight *Looks in his two foot tank direction*, i have only one in my three footer and he is getting bigger, but as sailfingirl said, he goes up to the water line and turns back, he does come out of the water on RARE occasions, but he does not leave the tank, and as sailfingirl said i dont think they like leaving the water, because he dont stay out of the water very long, and they can be quite fast too, i have plenty of openings that my snails can get out of, but they dont come outside of the tank, so i dont see you needing to worry about them
If you do start to get a infestation, not saying i would do this personally because i think the snails are adorable, but you can buy whats called Snail control but in better word it is called Snail killer, and that would bring your infestation to a end, thats only if you want, i wouldnt do it that way myself, i would take them to a local pond or something and put them in there