Hello Steed. I've added fresh, washed, egg shells to my tanks for a
calcium. Here are two links on Mystery Snails that will also tell you other ideas for adding Calcium.
Question: Mystery Snail Care? Mystery Snails
Also, some members have actually added calcium tablets for human consumption and it's done quiet well. Also feeding the snails fresh zucchini will give them more calcium.
On another note...your
pH levels really need to be above 7.0 or the acidic water will eat at the snail shell.
Best of luck!
Ken