Something around 500 will do. They lose all of their coloration and finnage when older ending up a silvery, kinda round fish. Looks kinda like an overgrown discus. When I was in Vegas this summer, the one hotel had 2 in a 20,000 FO. They were both around 20 inches long. For an easy referance as to what they look like when older, they are realted to the spadefish and look just like one without 2 of its stripes. Good luck I really like these guys too bad I don't have a tank big enough! You probably have either a tiera(Platax Tiera) or orbic batfish(P. Orbicularis). The pinnates(P. Pinnatus) are rarely found in captivity and die usually quickly after importation. For good pictures and ID of fish, the book "Indo-Pacific Coral Reef Field Guide" has good pictures but no info. It is a diver's field guide and is useful in IDing unknown species of fish and inverts.
-Big Dave