http://www.amonline.net.au/fishes/fi...h/cmertens.htm This is the fish you mean? I've never had one, and wow is it beautiful.
There is a butterfly called a Yellowtail (Chaetodon xanthurus), that looks almost identical except it requires at least 55g.
I know you have some SPS, and LPS in your tank alread. Do you plan to fill it with SPS, and LPS later?