No, shells decompose rapidly and dissolve, just like corals etc.
Even in Salt water...........and Ca++ levels at 400ppm............

Still think it's because of low Ca++ levels?

Living animals and algae use OH's to preciptate CaCO3 on to surfaces in definded patterns typically, once the animal/alga dies, this build up no longer occurs and the material is dissolved back into solution.

Freshwater snails, calms, mussles etc have a protective layer since they have a much larger gradinet than their salt counter parts.

If the layer is damaged for any reason, etc, then the shell will get "pitted" around the areas where the layer has been penatrated.


Regards,
Tom Barr