Photo copyright Tim Nicholson.
Taken in the Egyptian Red Sea.
Glassfish or Sweepers, Parapriacanthus ransonneti

Glassfish congregate in enormous shoals in wrecks and caves, liking the dark. Members of the small Pempheridae family, they feed at night on zooplankton.

Further Reading
Coral Reef Fishes, Indo-Pacific and Caribbean , by Ewald Lieske and Robert Myers, Harper Collins
The Red Sea in Egypt, Part I, Fishes, by Farid S Atiya, Elias Modern Printing House, 1994