Don't try the krill. Krill is a slippery-slope. Yes, many fish take to krill right away, however, there is real concern the fish may become dependent on krill solely eating that. Krill doesn't have much in the way of nutritional value, and is associated with lockjaw in lionfish and puffers, and I can only imagine doesn't do groupers any favors. NEVER feed freeze-dried krill (the type that comes in a can like flake food) to any marine fish. If you still want to try krill, make sure it's the frozen whole krill.
Teach the Panther to eat sliversides from a feeding stick or tongs. He will associate the stick with feeding, and then slowly begin adding variety to the diet. Start with silversides, many LFS's also have lancefish (larger sliversides), then try like sand eels from an Asian market, finger mullet from a bait shops, etc. Then start adding food you want the Panther to have jumbo shrimp, scallops,. etc. in with the sliversides. He might accidentally eat both, and slowly learn that the other is also food.
Groupers are sight-hunters, so I bet a lot of the reason he'll only eat silversides, is because that's what he's used to eating.
If you don't plan on very many other tankmates with the Panther, like one or two, the 120gal should be a decent home for the Panther for a long time. They are one of the more active groupers, but still isn't anything like a tang or angel.
