Snail shells are actually made of Calcium Carbonate. So therefore in the name, Calcium, and Carbonate, both GH (calcium) and KH (carbon) values would determine their shell health (in turn, whether they can survive). The outer hard shell layer is made of calcium, while the inner soft "shell" layer is composed of carbonate.
If you get whitening of the snail shells it's a calcium deficiency. Or possibly too acidic water, which would also dissolve the calcium layer and the carbonate layer would be compromised as well.
Look up "Ocean Acidification
" (you can add in "calcium carbonate
" in the search as well).
Here's a decent article Ocean Acidification | Smithsonian Ocean Portal
Yes, it does refer to sea water, but the same acidification principle can take place in freshwater as well, even with co2 injection. Pretty much co2 as we know, chemically reacts with the water, creating carbonic acid, which ends up increasing the amount of Hydrogen ions and lowering pH. Snails use calcium ions to bond with carbonate to create their shells (calcium carbonate), however those hydrogen ions (H+) have a stronger/faster bond with carbonate (creates bicarbonate), and with the increase of H+ they take away/use up the carbonate that the snails need to build/maintain their shells, ultimately leading to deteriorating snail shell health and possibly killing it.
With too low KH/pH/too acidic water, you could see snails' shells dissolving/eroding/pitting, good chances of dying if not placed in suitable water.
Here is a rather easy to understand basic chemistry explanation of aquarium water.
Not everything can live happily the same water conditions.