I don’t think it’s a hardness issue - we have a well and so generally get rocks out of the tap. Maybe a need for more food, as the algae has been a bit better controlled lately. I also note my pest snail population has tapered off in this tank, much to the chagrin of the pea puffer that lives upstairs. I’ll throw some zucchini in there and see if that helps.
Seems this is a common thing. We lost a prior mystery snail to a foot. My daughter stepped on that one when it escaped and we tried to nurse him back to health for a while, but he was never the same. I’m not sure my daughter will ever be the same either - it was quite traumatic. We have most escape routes sealed off now, but this one still managed to sneak out.