I'm a big fan of adding a few of each and then seeing how your tank responds. Different snails eat different algae. Nass snails eat junk from the sand (mostly). They aren't algae snails. Ceriths will eat algae from your sand and glass, but I rarely seen mine on the rocks. I like Trochus snails over the Astreas, and they will eat it off the glass and rocks, but I never see them on my sand.