The Moorish idol is a tropical and sub-tropical marine fish that lives in the Pacific Ocean. Zanclus cornutus has become a sort of "impossible dream" for aquarists, who love this fish for its striking appearance and distinct behavior, but find it almost impossible to keep alive in a captive setting.
Moorish idols are most notable for their vibrant coloration and graceful form. Triangular in shape, Z. cornutus is much broader from dorsal to ventral fin than between the pectoral fins. The dorsal fin is thin and elongate, arching over the back of the body past the caudal fin. This delicate dorsal fin, called a filament, is the inspiration for Z. cornutus' alternate name -- the crowned scythe. This lengthened fin is sometimes called a philomantis extension. They typically reach lengths of 7 to 10 inches and their scales are white to yellow with thick vertical black stripes. Small, bony nobs develop in front of each eye in adult fishes and grow larger in the males. The moorish idol has a long, pointed snout. Inside the elongate mouth are bristle-like teeth. The bright orange patch on the top of the snout is one way Z. cornutus can be differentiated from its doppelganger, Heniochus diphreutes, often referred to as the false moorish idol.
Native to the Indo-Pacific and Pacific Oceans, the moorish idol dwells among the coral reefs in shallow ocean waters, usually along coastlines. Moorish idols prefer hard-bottomed habitats in depths ranging from 10 to 600 feet. Its geographic range is fairly widespread along the eastern and western Pacific, and some specimens have even been spotted off the coast of Florida, although those are assumed to have originated from human release rather than natural inhabitation.
The moorish idol grazes on tunicates and sponges living on coral reefs and anchored to the ocean substrate. They spend much of their time nibbling food, and are highly active swimmers. Z. cornutus darts, loops, and speeds through the ocean, carried by swift currents. Moorish idols are often seen travelling in pairs and in rare instances, small schools. Although it looks elegant and graceful, the moorish idol can be aggressive toward other fish and will stand its ground in a confrontation.
Keeping Moorish Idols in Aquariums
The only way to keep moorish idols healthy in captivity is to replicate their natural reef environment as closely as possible. Aquarists determined to keep Z. cornutus often come to terms with the fact that a tank containing this species must be designed around it and its needs. A reef aquarium should be fully matured and established before a moorish idol is introduced to it. The minimum tank size for this fish is 200 gallons, but the bigger the tank better. These fish need lots of rocky structures with long swimming paths to replicate their natural environment. Moorish idols are omnivores and providing them a varied diet is key to their ongoing health. Tiny organisms on live rock, seaweed, mussels and clams, cultured sponge, and angelfish food containing sponges are suitable food sources for Z. cornutus. Machine-produced wave motion, highly oxygenated water and pristine water quality are minimum requirements for the moorish idol's captive survival. Even after addressing all the parameters set to keep moorish idols, aquarists sometimes find there is an X factor missing from the fish's diet or habitat that they cannot replicate, which leads to the fish's demise.