Hi,
I noticed that when the salt level drops a great deal snail do not move at all. When I add salt slowly to bring up the gravity, snails starts to move. If your gravity is 1.023 and it drops down to 1.020 or below in such a short time, the snail will react to this. I check all my tank every week for gravity and water levels. I always have all my tanks at 1.024 and they stay this way even when I make water changes. I have snails as old as two years old in most of my tank. I'm not saying I don't lose any I do, but it's rare to see one dead on the ground. I place a small non-toxic dot on the snail to know how long it's been in the tank.
In all my tank I must have at least over 100 snails, I just brought five the other day to be place in a new aquarium for dwarf seahorses.
Good Luck!