Dear hobbyist,all yellow tangs have a white lateral line on their bodies after the gill plate that extends towards the tail . At night, while yellow tangs are asleep ,the lateral line on their bodies becomes more pronounced until the yt awakens and becomes active within the aquarium .My hippo tangs' lateral line fades while it sleeps from black to light gray . And when its awake, its color returns .Don't be alarmed ,this is perfectly normal .