Most of the snails which eat algae will feed whereever they find it, bottom or not. The only truly substrate oriented herbivorous 'snails' are adult conch's, i.e. Strombus alatus, but they grow much too large for a 10 gallon. A better bet would be the Cerith type snails which will graze on detritus, brown diatom 'films' but not filamentous algae, according to the Fenner 'Reef Invertebrates' book. I don't think that there is a gaurantee that they will stick to the sand bed however.
