therer is a product called HAD A SNAIL that will kill the snails in your tank. sounds like you must have had 1 snail and that it all it takes. they can reproduce alone..follow the directions on the container and  you should become snail free... also if you do not want to use the  product i have suggested,, if you put   a fish called Clown Loach in the tank it will eat the snails.. get a couple of them...good luck