Do have any species in particular in mind? Of the two most commonly offered, the Emperor and the Threadfin, the Threadfin is the much better choice, it will adapt it's size better to a 200, reaching probably 1 1/2 to 2 feet. The Emporer, might reach over 3 feet, obviously way too big for a 200. The threadfin also has much more of a 'community' personality, vs. the Emporer, which will eat anything it pretty much wants too.
There's one more sometimes offered, the Macolor niger, I would just skip, juvi's don't have the best track records and they get really ugly as adults.