Lionfish says.......
As cool as I find Groupers to be, they really couldn't ever get a good spot in a reef. Of course a reef tank with a grouper wouldn't be a "Traditional" reef. I say this because, and this, in my own opinion, because to me a reef is a tank with lots of little inverts running around and corals and small colorful fish. Now, no matter, it could be done of course, however, it would have to be a very large tank and filtration would be the major issue at hand. You would need a major filtration system since Groupers are notorious for being sloppy eaters and leaving an incredible amount of waste. This means, ammonia, nitrite, and nitrate will be sure to rise in a quuick manner. The waste would settle on corals and they would eventually die unless a very good powerhead system was used. Also, small fish would not be possible to mantain with a grouper. And inivetibally inverts would be almost non-existent after awhile. I recommend groupers to only be in fish only/aggressive/predator tanks. Hope this helps.
