The yellow tang (Zebrasoma flavescens) is a saltwater fish species of the family Acanthuridae. It is one of the most popular marine aquarium fish. Adult fish can grow to 20 centimetres in length, and 1–2 centimetres in thickness. Adult males tend to be larger than females. Yellow tangs are bright yellow in color. At night, the yellow coloring fades slightly, and a prominent brownish patch develops in the middle with a horizontal white band. They rapidly regain their bright yellow color during daylight. Male and female yellow tang look very similar. When mating, however, males change color and have a "shimmering" behavior which makes them identifiable.(wikipedia)