This behemoth is a Goliath Grouper (Epinephelus itajara), the largest fish commonly observed on the reefs.  This fellow tends to hang out on the sand flats at Mixing Bowl and can often be found lurking in the numerous caves and tunnels above the wall.  Even though this species grows to a maximum length of 8', this particular fish is only about 6' long.  I haven't measured him with a tape, though, because he tends to run hide when I get closer than about 5'.