Well, you could do either these things:
Adding a handful of crushed coral into a mesh bag and hang it on the side of the filter, or add it into the filter itself. Now, this will only be safe if your pH is also low, crushed coral will increase your pH, so, if you already have normal ~ high pH, I wouldn't recommend it.
Marine liquid calcium would work either, be careful with this, and don't overdose.
Vegetables that are high in calcium like spinach, blanching them and letting your snails feast on them will work too.