They get two and a half feet long...even if you did have room in that tank, you'd have to put something almost as large as the panther is or he'd eat it. They're shameless in regards to the size of their food. Mine (About 3-4 inches long at the moment) eats comet goldfish almost as big as he is.
However, were I you, I really would look into getting a larger tank or getting rid of the grouper. They're gorgeous, they're very interesting to watch...but they grow faster than any fish I've ever encountered. There's a reason that guy wanted to get rid of it. If you don't get a larger tank, it will very likely die...after suffering a miserable exsistence.