How old is your tank? What are the specific water parameters, especially ammonia, nitrite, nitrate, pH and specific gravity? Other fish in the tank?
IMO, no feeder fish long term :nope: but just to get them interested, but not as a long term diet. This is not healthy for them, and it is unecessarily expensive too...not to mention, have you taken a close look at the health of feeder fish?
1) I would take the panther back now...if the store will take them. A wonderful ploy the panther grouper is...small and cute...but grow fast and become very difficult to trade in at larger sizes. You can not put any other small fish in with it, BTW, and it will quickly outgrow that tank.
2) Was it eating at the store?
3) Try ghost shrimp aka paleo shrimp...for now to see if it will be interested. Also try krill, shrimp, squid and silversides (frozen)...definitely not a flake eater.