Poor condition of water snails

I have had a small wildlife pond for about 7 years and have always had water snails (originally introduced). The snails are prolific with young ones every year. However, the snails over a year old all succumb to whatever causes the periostacum (had to google that) to be lost. So the older ones look quite rough and
white and I suspect don't last long once their outer layer is gone.
Has anyone any idea what causes this? I have, in the past, added oyster grit to the water and cuttlefish in case it is a calcium problem, but these additions made no difference. sorry I haven't added the picture very well - a novice at the computer!

Has anyone any idea what causes this? I have, in the past, added oyster grit to the water and cuttlefish in case it is a calcium problem, but these additions made no difference. sorry I haven't added the picture very well - a novice at the computer!
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It's not a problem I have come across in any of my ponds.
Just thinking of my snails indoors, they look very white and dry sometimes, then they brighten up.
Wisdom is not putting it in a fruit salad.
I saw the question and wondered how long they live - the wonders of Google!
I'd have had to cycle to the library to find that out when I were a lad
Wisdom is not putting it in a fruit salad.
It's too easy now isn't it @Pete.8
I live in west central Scotland - not where that photo is...