if you have no animals in your tank you could try using dry ice in a container of water set in the tank so it fills up with CO2 and then close it up. Orchids would love this temporary boost in CO2 and snails would not.
They're probably after the algae on the plants and surface of the medium. Getting rid of them is pretty easy as the above posts suggest. Also, you can use Sluggo (a snail and slug bait which is Feric Oxide). Hope this helps.