Holacanthus passer is a deep-bodied, laterally compressed angelfish from the tropical Eastern Pacific, reaching roughly 14 inches at maturity. Adults wear velvety dark blue-gray coloration with blue-centered scales, a narrow white vertical bar across the flank, yellow tail and pectoral fins, and trailing dorsal and anal fin tips. Juveniles look entirely different, presenting orange to brown bodies crossed by several narrow blue bars along with one contrasting white bar. The species occupies rocky reefs and coralline areas from the Gulf of California region south to northern Peru, including offshore islands such as the Galapagos, Cocos, Malpelo and Revillagigedos.

Adult housing should be generous and mature. A 180-gallon aquarium is the minimum for a full-grown specimen, and larger volumes are better suited to a fish of this size and activity level; some published guidance argues for 250 gallons or more. Build the layout around abundant live rock arranged into caves and crevices for shelter, while leaving substantial open water for continuous cruising. Excellent water quality, thorough filtration, strong oxygenation and areas of appreciable current all suit this rocky-reef inhabitant. Established live rock does double duty, supplying grazing surfaces and micro-invertebrates that supplement a specialized natural diet, so an aquarium with some age on it is a far better candidate than a newly assembled system.