Tuxedo Urchins keep dying

I've recently had my entire tank wiped out. I have resolved the issue which I was told was too much calcium by the LFS who I trust. All seems good now with the fish. i started with two clowns and Banggai cardinalfish which seem to be doing fine along with many hermit crabs and snails. My first tuxedo urchin died when all else looked fine so I tried another. The second recently died as well. Is this a common occurrence or something to be concerned with? LFS tests my water and says all looks good.