Tank size does not permit Clown loaches, or many other loaches. some of the really small ones may work, as will a dwarf puffer IF there aren't many other fish in the tank already. None of these options are real good if the tank is already stocked up. Hang a piece of cucumber in the tank overnight, and then remove it in the orning. it will be covered with snails. do this several times and you should be able to get your population down. if you want them gone completely. smash any you see after you take out the cucumber. within a couple of weeks you should be able to eliminate the population.
Dave