My yellow tang has always changed color at night. He seems healthy, eats a lot & looks good during the day, but when the lights are off he turns a brownish color & the pale line along his sides gets very white. As soon as the lights come on in the AM he's bright yellow again. I have never seen or heard this talked about ANYwhere, & I wonder if it's normal, or if there's a problem. He has always been a slightly jumpy/nervous fish, but seems OK. Is this a problem? Maybe a defense mechanism for eluding predators on the reef at night?