They use calcium for their shell.
If the pH of the water is below 7.0, the shell will dissolve over time and they struggle to live. If the pH is above 7.0, the shell won't dissolve.
Calcium supplements are only necessary if you keep them in acidic water (pH below 7.0), or soft water (GH below 150ppm).
What is the GH (general hardness), KH (carbonate hardness) and pH of your water supply?
This information can usually be obtained from your water supply company's website (Water Analysis Report) or by telephoning them. If they can't help you, take a glass full of tap water to the local pet shop and get them to test it for you. Write the results down (in numbers) when they do the tests. And ask them what the results are in (eg: ppm, dGH, or something else).
