Been doing some reading from www.reefkeeping.com
Here are some extracts from the article on snails
"Snail tissues often seem to be composed of thin layers of tissue that are very filmy and diaphanous. Consequently, they often suffer significant damage during water changes or during transport from a dealer's to an aquarist's tank. The circulatory system of snails may be very complex (Figure 5), and many of its vessels and channels can rupture under stresses caused by changes in salinity. The vessels in the kidney are numerous and delicate, and may rupture if the animal is not slowly acclimated when being moved from one set of water conditions to another. If the acclimation is too fast, the animal will die in a few minutes to a few weeks. If the snails are drip acclimated, the acclimation time may need to be on the order of five to ten hours for maximal survival."
"Many of the actual individuals of Astraea offered for sale in the reef aquarium hobby come from the cooler waters of Baja California. These animals are not from areas that have sand substrates or, really, much of anything else in the way of flat surfaces. If they get dislodged from rocks, they tend to fall into crevices or crannies where they can reach a rock with their foot and attach to it. Consequently, they have never developed a "righting response." This means that if they fall from the aquarium walls onto the sand, they will not be able to turn over, and will die there unless somebody or something turns them over. Given that they also are not warm water animals, they tend to die young and leave a good-looking corpse in the aquarium."
No wonder some of my astreas are dead..blamed the hermits for nothing..