Looking for help with my Black Tang

I have had him for a little over a week and he was eating and picking on the rock. When we first put him in the tank he was very skinny, but now he is fat. The last two days he has not eaten anything and is acting different. He is not hiding but just seems to hang out in one spot. He is not breathing hard and does not show signs of stress. Any suggestions.
 
What type of food / algae are you feeding the tank? Most of the tangs I have had eat off the rocks for a few days when introduced, but then start eating the seaweed clips and spirulina food we feed.
 
I have been feeding seaweed on clips (green,brown and red),spirulina food as well as Mysis shrimp. He is in there with three other tangs (Chevron, Atlantic Blue, and a Desjardini Tang) all of which are eating and fat. The tank is 300 gallons.
 
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