OK this is just my opinion
but
I read my book and it says for leopard groupers to "feed 2-4 times per week, meaty foods such as marine fish or crustacean flesh"...
1. test water to be sure all parameters are good (Ph, salinity, ammonia and trItes)
2. turn your lights off for one whole day
3. thaw frozen (meaty rations) completely
4. turn lights back on
5. feed (other fish) flakes or pellets
6. once others are done eating scatter in grouper rations and observe
if he doesn't eat he is stressed
2 questions, how did you acclimate and how did he eat during the first feeding