| the snails will come into your tank via anything new you may of added recently, or even as eggs attached to anything new you've added, not so recently. usually they come in by new plants you may have aquired and added to your tank.
lucky for you though there are many dif. types of fish you can add to a tank to take care of the problem, depending on what type of tank you have.
i have a freshwater community planted tank. they are all small fish. in my tank i have 2 yo yo botias that ate an entire festation of snails in what seemed like just a week! it was awesome to not notice them anymore
any fish of the 'Botia" species (clown loach, kuhlie loaches) will eat them (community tank, small fish), many dif. cichlids will also eat them too if you have bigger fish that would go along with the cichlids without anyone getting hurt. As far as shrimp go I've also kept ghost shrimp in my community tank and they will pluck the little babies straight from any snail eggs that are planted around in the fishtank & eat them |