Nerite Snails - Useful Link

My older nerite - purchased in May 2017 - looks as though he doesn't have much longer.  He's really lost his suction and has been lying on his back every single morning for the past month or two with his innards hanging out. I turn him over, but, about 50% of the times I look in the tank (now generally restricted to lunchtime and a couple of times in the evening during weekdays), he's upside down on the tank floor again.

Yesterday, I found this interesting link which has more info on it than I've read elsewhere https://www.vivofish.com/nerite-snails/ (there is a typo where 'nitrites' should be 'nitrates' which I've contacted them about).  I'm wondering if the oedema experienced by elderly nerites is what accounts for this - it's as though his innards are too large for his shell.  As a consequence of reading this last night, I've left him on the tank floor today to give him a chance to rest rather than turn him over - fortunately, I don't think the fish bother him.  Most of the time, he seems to land on top of the flat stones surrounding the tank so, assuming he's still alive (just gone to check and the light coming on seems to have woken him and his feelers are moving), I plan to take them out during this afternoon's water change to prevent him cracking his shell any more than it's already cracked and at least give him a softer landing.

Well spotted!