Adult. The adult Emperor Angelfish is commonly referred to as the Imperator Angelfish. With its beautiful blue body covered with bright yellow horizontal stripes, it makes a captivating addition to any tank. The changing angelfish will vary in coloration and pattern as it transforms from juvenile to the adult coloration. It grows up to 16 inches and needs to be kept in a tank not less than 150 gallons. The adult Emperor Angelfish needs plenty of live rocks for hiding and grazing, that is why make sure to decorate the aquarium cleverly. It also requires extensive rockwork and deep caves to feel secure. The adult Emperor Angelfish may found nipping at stony and soft corals, clam mantles, but doesn’t disturb other small polyped stony corals. It should be fed a diet of Spirulina, marine algae, high-quality angelfish preparations, mysis or frozen shrimp, and other meaty items. The adult Emperor Angelfish thrives well in a temperature range of 72-78 degrees Fahrenheit and pH of 8.1-8.4. It is slow to accept captivity and very shy in nature. The adult Emperor Angelfish is susceptible to a variety of parasites and proper quarantine procedures are then recommended. It is not considered reef safe though some specimens have been reported to live peacefully without harassing small-polyped stony corals.