Millet butterflyfish stripe runs from above the eye to the base of the operculum, and there is a large black patch on the caudal peduncle. This fish is endemic to the Hawaiian Islands and the Johnston Atoll, and is found down to depths of at least . Its habitats include coastal fringing reefs, outer reefs and lagoons. The millet butterflyfish is a schooling species found in mid-water that feeds on zooplankton. It also feeds opportunistically on polychaete worms and other small invertebrates on the seabed. Food is drawn into the mouth by rapid suction. It does not seem to be territorial but
Copella (genus) observation. The pectoral fins are positioned immediately behind the operculum or gill cover, while the pelvic fins are further back along the body, about midway between the head and the caudal peduncle. The dorsal fin is positioned above and slightly behind the pelvic fins, and midway between the pelvic fins and the tail fin is the anal fin. The tail fin is forked in shape. Males usually possess more elongated and ornate unpaired fins than females. In the case of "C. arnoldi", the tail fin of the male is somewhat asymmetrical, the upper lobe being larger than the lower lobe,