did you put the snails in yourself or did they just appear?
if they just appeared, it'll be because of live plants which may be in your tank, snails lay eggs in the plant which then hatch in your tank once you have got the plant home and put it into you aquarium, thats 1 main reason why i dont use live plants.
and of course, because i have oscars
if you want to get rid of these snails, the way i done it was, place some lettuce just above water level inside you tank before you go to bed 1 night, and watch how many snails you have caught in the morning, obviously you'll have to do this for a few days to get the whole lot but it worked for me.