They are great for cleaning the glass among other things. I don't know a snail that cleans the glass more then they do. IMO
they are worth the buy just for that reason. Yes you might have a couple that die but more often than not they remain upright and make a great addition to the clean-up crew. Actually, I've had more die due to attacks by Nassarius snails then anything else. I never see the Astreas on the sand and I believe the potential attacks by the Nassarius snails are the reason.