i have the same problem the snails i have are tiny little brown snails
there a pia cause they like to live in my fx5 and they seem to clog up the sponges
you can kill them with copper, but it has its risks and im not that eager to try it
i never really see them in my tank either, but i have loaches and they probably eat them, any way you might as well live with them cause its almost impossible to get rid of them