This article is from the Aquaria: Disease, Algae and Snails FAQ, by Elaine Thompson, Thomas Sasala and George Booth
Most snails do best in harder/alkaline water. If the hardness/ph drops
below a certain point, their shells will start to dissolve and/or grow
improperly (the behavior seems to be based on species). Malaysian
trumpet snails seem the hardiest, showing little adverse effect from
soft water. The Ramshorn snails shell will start to dissolve, and gaps
will form in the new shell growth. Mystery snails will form gaps. Most
of these problems can be corrected by hardening the water, and the
snails will recover, although exterior shell damage (from dissolving)
will remain.
 
Continue to: