My Kole did that for a while. Tangs have the ability to change their color tone, lighter and darker, depending on stress etc.
My Kole would lighten a lot early on in the tank and I finally figured out that he was chasing his reflection in the glass and that was stressing him. After he settled in, his color now stays dark most of the time. I have had him for about 3 years.
If yours is eating well and active, I would not worry.
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Hello, my name is Joe and I am a reef addict.
......Hi Joe..........
Current Tank Info: 120 gallon mixed reef with TBS live rock, 8 X 54 watt TEK T5's, Euroreef RS250 skimmer
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