Freshwater Calicum Supplement - Apple Snails

[High tide]

Cuttlefish bone or two should work. Made of Aragonite, totally safe, and dissolves readily. They float so you'll need to anchor it down or float it.

Part of the problem could be your top up/water change water. Do you know what the calcium/alkalinity levels are? If the levels are low you may need to bump them up with supplements. If they are fine it would probably be good to do more frequent and larger water changes.