kitch40

Need parameters, color change is usually stress related though.

Yep... run every test you've got.

Temp, Salinity, Ammonia, Nitrite, Nitrate, Phosphate for sure. Got more? Even better! :-)

Also list anything new that has happened in the last week or so. Anything added to the tank... fish, additives, inverts... list anything.

Thanks for the help, The tang hid out last night and today,he seems normal, no colour changing or such. He has always been the most boisterous fish in the tank as well as the largest. Anyways I did a water test and the parameters all seem good, although when I did the water change I forced the salinity up a bit from 1.0245 to 1.026. The temp was about 80f and it starts at 76f every day.
Temp 76, ph 8.2, nitrite 0,nitrate 5 mg/l, phosphate 0, Kh 9,Calcium 440, Magnesium1420,Salinity 1.026 Thanks again Kitch40

Tangs can change colors based on their mood. I've seen my Hippos look dusky, then later vivid. My Naso changes from ghostly white to nearly black end to end. Yellow Tangs can look bleached, then bright yellow to almost orange later.

If the color stays one way long term, like what you described, as in day after day after day, then I'd look for cause. But what you see tonight could be completely different a day later, as in back to normal.

A skittish tang will swim just like you described when it apparently feels stressed. This should stop as well - perhaps it sees its reflection.

Thanks Marc. After the first night he seems normal, his black stripes had turned gray and he was just tearing all over the tank, but now he seems a ok. His stripes can change almost in an eyeblink!!!

Exactly. Like I said, it's almost like a mood thing.