Opahs are commonly known as moonfish
Opahs? I've never even heard of them.
They are fishes comprising the small family Lampridae
Hmm cool! What else do you know about them?
Opah
Opahs (also commonly known as moonfish, sunfish (not to be confused with Molidae), kingfish, redfin ocean pan, and Jerusalem haddock) are large, colorful, deep-bodied pelagic lampriform fishes comprising the small family Lampridae (also spelled Lamprididae). Only two living species occur in a single genus: "Lampris" (from the Greek "lamprid-", "brilliant" or "clear"). One species is found in tropical to temperate waters of most oceans, while the other is limited
gonorynchus" (Linnaeus 1766) (Cape of Good Hope)
Vertebrates Fishes Gnathostomata – jawed fishes Class Actinopterygii – ray finned fishes Order Lampriformes.
Family Lampridae – opahs 
- Spotted opah, Jerusalem haddock, moonfish "Lampris guttatus" (Brünnich, 1788) (all oceans but not in polar waters, occurs throughout South African waters, usually well offshore)
- Southern opah "Lampris immaculatus" Gilchrist, 1904 (circumglobal south of 30°S)
Family Lophotidae – crestfishes
- Unicorn crestfish "Eumecichthys fiski" (Günther