CS> My very large apple snail's shell eroded (from the tip at
 CS> first).
Are you sure it was erosion?  It's quite possible that your dempsey was 
nibbling on it.  I had a mystery snail in a young cichlid tank, and they 
nibbled his shell away trying to eat him.  I finally had to pull him out and 
place him in another tank.
Robby
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