Issues with adding larger fish (mainly tangs)

I have recently had an issue with adding larger fish to my tank. Right now I have mostly tiny fish with a 2.5-3 inch wrasse being the largest. I have tried to add 2 powder blues and 1 naso (not at once but 3 separate occasions) and they have died within 3 days. All other fish have done ok in the tabk. I assumed it was ich but speaking to my lfs he said he didn't think it would do so in such a short time. Is it bad luck or could their be something in my water? Params look ok, small fish and corals all doing well...