Shell erosion, pH and calcium
Help! My snail's shell is eroding.
Erosion in snail shells is caused by low pH. A snail shell is composed of 98% calcium carbonate  and this dissolves when pH is too low. The pH at which this will start dissolving is complex and depends on other environmental variables like temperature but typically in an aquarium environment this starts happening at <7.6 pH . The lower the pH, the faster the shell will dissolve.
So it's not caused by a lack of calcium?
A lack of calcium won't cause damage in existing shell growth. It only affects new shell growth and generally means the new shell will be weaker and thinner or sometimes grow deformed. Sufficient calcium is particularly important for young snails growing up and low calcium levels can increase mortality rate in them .
A zebra nerite (Neritina natalensis) snail with some shell erosion. The white pitting towards the end of the shell is indicative of erosion caused by low pH acidic water.
Ok so my pH is too low then. How can I raise it to prevent shell erosion?
A little shell erosion won't actually harm your snail, it just doesn't look very nice. Only extreme erosion where there's risk of exposing the flesh is a concern to their health. Generally pH of at least 7 won't cause shell damage of too much significance unless the snail is already in bad condition. To raise your pH higher, the best solution is to add a bag of crushed coral to your filter. Crushed coral is also mostly calcium carbonate so you will both raise pH and add calcium. If you just wish to raise pH you can add sodium bicarbonate (baking soda). All these take a little time to work so you'll typically needs days to weeks to notice significant changes .
Great! I'll do that. How long will it take my snail's shell to heal after I raise the pH?
Unfortunately a snail cannot repair any existing shell growth so any damage to it is permanent. The best you can do is to provide it with an environment where it won't get worse.