I went thru the very same thing the first couple months I started up my tank ( fish would die in a week or so but corals were doing great) . I think mine was a combination of a lot of things ( parasites , acclimation , WC
What I did was add a uv
sterilizer to the return line of my CF
and went fish less for about 6 weeks, and did a lot more research about acclimation.
Now I acclimate using the drip method for around 2 hours and I have not had any issues since.
Hope my 2 cents helps... KCCO