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I have noticed at night my yellow tang develops 2 white lines down its side on each side and identical in length etc.. After lights have been on for 20-30 mins it disappears
I would say it's just your tang. If it's eating and seems well I wouldn't worry. I have read that at night they lighten in color and at first glance in the morning they might look pale, and then regain their bright yellow color after a few minutes in the light. Sorry I can't be of more help.
those two stripes are normal. if the two white stripes show during the day that means he is stress/scared at the moment. it usually happens to my yellow tang when i stick my hand in the tank to move around the rocks or clean up the sand bed. ... the white stripes also appear when the tang is sleeping. my guess is that he was sleeping when the lights were off and suddenly the lights turn on. he needs time to wake up so the white stripes will still be visible for a few minutes