I purchased a handful of snails from my LFS (turbo, trochus, and margherita) and noticed over the next several weeks that a few of them (all three varieties), at one time or another, have fallen upsidedown and couldn't right themselves. Two of them died, which I'm presuming was from falling over and me not being home to flip them back over. Is this normal for snails?? I also have a lot of green algae on the sandbed (probably need to do more water changes), so they might be slipping on that. Any thoughts?