I have had snails thrive and also drop dead in both fresh and salt.
First, some are already large and older when you get them. I had a olive nerite as long as my thumb tip from the nail to knuckle when my lfs tossed him in with some plants. He was moving around my 29 gallon and did great for 5 months.
I added a large cherry sized mystery snail and medium ramshorn after my original nerite and they all lived together for about 3 or 4 months when the mystery died, then the nerite in a few days, then the ramshorn the next month.
Params were spot on, in fact that may have been my problem, all my plants were thriving and my only fish was a small male betta, honestly the snails were the bulk of my tanks bioload and without noticing it my tank was plum out of algea! no wasted food. I caught them munching on my smallest and favorite crypt parva carpet before they drank the kool-aid.
I felt lousy. Later I forgave myself because it was my first tank, and now I have a pretty big crew of salt water snails of various shapes and sizes (my snail united nations cabinet) and they occasionally do keel over, sometimes I guess it was this or that, but usually I can't know as the others are still happy and there are no clues. Twice actually, one asterea (spelling im sure) and another margarita snail. still have another surviving 3 of those 2 types in my 55 fowlr and a narcissius (spelling again lol) snail who is teeny tiny. And a giganto sand sifting snail the size of a chicken McNugget! he rules.
OKAY this is the point of this babble: See a dead-looking snail? Not responding?
Here is what I do... make a note or mental note of the first sighting of the "dormant snail"
If it looks like it is overturned and can't reach any surface to right itself, I use a tool to roll it over.
24 hours later................. Any movement?
another 12-24..................I reach into the tank and take it out briefly to inspct, sniff the shell if you don't see any body and you will know
