Smaller snails my have trouble with larger pellets or fish food - algae based flake may work better. But, most snails are designed to scrape films from surfaces. You can try to get some dry sand dollars (or SW safe small tiles), wet a piece of nori, and apply it to the sand dollar/tile. Dry it in a toaster oven on low for a few minutes. Place the whole thing in the tank for snails (or fish) to graze on. Then just remove when cleaned.