Ring Cowries - any experiences?

[Myka]

I bought five Ring Cowries to add to my tank, partly for something interesting, and partly because I hear they are good at getting the tough green algae that grows on the glass in those little specks. I lost two of them within the first few days, and I lost a third a week later. The remaining two seem fine, but I'm wondering if they are really sensitive or what? Also, they sure don't seem to do very much! Any experiences with these guys?

PS They were in the bag for 48 hours, but they stayed a good temp, and I did acclimate them very well.