Like they said, its a trumpetfish - either Aulostomus chinensis or A. strigosus. The reported maximum wild adult size for either of these species is 30", (another species gets larger) but I've never seen one that large, and they won't get much longer than 20" to 24" in captivity.
The primary difficulty is feeding them. Without a lot of work, they are difficult to wean off of live food, and it is hard to get enough marine feeder fish to keep them satisified. In the dim recesses of my mind, I seem to recall the "chinese dragon" as they are sometimes known as in the trade has something to do with the flap on their lower jaw....
Jay