a new rock wouldnt kill a fish all to quickly. perhaps create another cycle? yes. unless you had some serious toxins in a rock i say doubtful. i would honestly take your time. i wont add corals till i have coraline algae growing myself. i let me 180 run for a few months with all rock in place and settled before i rushed in to kill things. these guys are live aminals =). wait till all your bacteria blooms/cycling is done before introducing live inhabitants. although lots of LFS love it bc they get to sell you more fish. i would create a plan of what you want to keep also for corals as there are dif needs for certain kinds. softies, lps, and sps all tend to have a different demand for lighting req., water params (dirty tank/clean tank), etc.

funny enough i purchased a rather nice large specimen of yellow tip elegance (rare), i figured hey this guy knows what he's doing. he has tons of frags. after it receeded and died, i noticed the undercut of the skelleton was so close he hit into the polyp and it was doomed from the start. in the case of your fish, i say stress. how long did you acclimate him? and how was it performed?