Sorry about your bannerfish.
Bought mine at about 5 cm, kept it in a self made fish trap, slowly trained it to feed on frozen Brine Shrimp, and when I was sure that it has started to eat for a week or so, I freed the fish from the trap. It has since grown up to about 15 cm now on a diet of frozen Brine Shrimp and frozen Mysis Shrimp.