Zanclus cornutus is common at Kwajalein on most lagoon, pinnacle and seaward reefs. Outside of Kwaj, it has a wide distribution, ranging all the way from the eastern Pacific to western Indian Ocean. It is the only species in its family, and its primary food is sponges so it does not do well in captivity. Usually in pairs along the reef but large ones occasionally form schools.
The photo below was taken in Hawaii, but it shows that they don't seem to care about the difference between the floor and ceiling of a cave.
Night colors on a sleeping fish.
On the rare occasions when they form schools, all the participants seem to be very large individuals with long trailing dorsal fins in good condition. Usually they are swimming almost inexorably along the reef, not stopping to feed or fight. Sometimes if a diver gets in front of them, it will confuse them and they will mill around for a few minutes, as in the first photo below.
Created 8 July 2017