What did the store tell you about the Panther grouper?
Fact is they get HUGE - well over a foot if I recall, and will eat nearly any other fish you try and put in a "reef" tank. They grow quite rapidly...I would return it while you have a chance, as many stores will not take them back once they are larger then a few inches. Nasty little trick.
Brine shrimp is not sufficient for a panther grouper. They need meaty foods like silversides (frozen), krill, shrimp, squid, etc.
Such fish are not commonly put in a reef tank because they are "messy" fish if you will - a predator that eats a lot and produces a lot of waste. This in turn results in poor water quality and causes corals etc to suffer.
Panther groupers are typically housed with other predatory fish such as large lions, triggers, large wrasses, large angels, puffers and eels.