If Im not mistaking, the calcium will cause a PH change in the water. Although, only a couple of sand dollars in a decent sized tank will "probably" not make a whole lot of difference If it doesnt fall apart in the water.
Calcium will increase the hardness of your water. and could adversly effect your enviroment. You might be able to get some water safe seal on them. But personally I would save them for that salt setup you've always wanted.
(you know you want one)