Assuming you aren't trying to suplement anything in particular, Blinky is correct. snails can actually live on bio-film only long term (or at least the ones under my RFUG plate seem to). They will eat virtually anything, and all of it they can find. If you want to add some high calcium stuff to their diet, Hikari Crab cuisine is great for them, so is a piece of cuttlebone tied down somewhere in the tank.
Dave