You can always drop a piece of lettuce into the tank. Pest snails will find their way to it, you pick up the lettuce and dispose. Another way is to just pluck them off the tank as you see them and either crush them or do as I did and put them in a soda bottle (sealed) and dispose. If you have the space, a clown loach will be a very effective snail remover, more than an assassin snail which can only eat snails larger than its (what's the word? The thing it uses to get into the other snail's shell?)