Can I add a calcium supplement instead of salt for mollies and fiddler crabs?

I have a 10 gallon with 2 mollies, a fiddler crab and a red tail black shark. Can I add a calcium supplement instead of salt to the tank? I do not want to kill the RTBS and have heard that adding a calcium supplement will give the mollies and crab what they need to survive. thanks for the help. PETCO sucks and cant answer anything