I'd say you are right about your tap water and the nutrition your snails are getting that's causing problems.
What's your tap water hardness? pH, gH, and kH? API sells some good ones individually, if you don't have the test kits; (be sure to look for the drop test kits).
If your pH, gH, and kH are really low, then that could be the root cause of shell erosion.. also what you feed them is important. You have to supplement their diet with ample amounts of calcium as well as vegetables, algae, and etc to get their shells in top shape.
For now, I would test out the pH, gH, and kH. Feed them fresh high quality veggies and even some pieces of Tums will greatly help prevent shell erosion; however, what has been done is done... They can't just simply repair their shells with new colors instantly at all, but you can make sure their new shell growth is stronger by making sure you feed them well.
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