Need help keeping new fish alive.

[hummer]

Not sure why but all the new fish I buy die within 3-4 weeks. I've tested the water and nothing seems wrong. I acclimate new fish using the guidelines on the JL site. I have a 90Gal tank w/ 33Gal sump holding 4 clowns, 1 blue damsel, 2 cardinals and 1 blue tang. The blue tang is about 3-4". The latest fish that died was a 3" salfin tang and a 2" rabbitfish. Nothing seems to bother the old fish though. Any advice would be appreciated.
Thanks