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Greetings, I am Dr. Pat. I have worked with aquatics and invertebrates.
First, are there problems with the snails? Do they seem to have issues with shell or integument?
If they will eat vegetation, one of the best sources of bioavailable calcium are fresh, clean, dark green leafy vegetables. You can rig up a safe plastic feeder to hold the leaves and remove it after several hours. The animals will learn when feeding time is and you will be able to control any waste or excess material without damaging the chemical balance in the water. The fish may also feed on these greens.
Adding anything to a balanced tank has risks. You can change the hardness with cuttlefish/bone in the water and these sources are not readily bioavailable anyway. fresh leaves can be removed.