Dark patch/skin issue on yellow tang

[Gcowles]

I recently purchased a yellow tang, and QT in copper for 1 month before adding him to my display. He was out and eating well for about a week. On Monday however, he was hiding in the rocks all day. The past few days he has been hiding the entire time, but will come out to eat brine shrimp. There is a dark patch behind his right pectoral fin and gill, and on his dorsal fin. I’ve attached a picture of him on Sunday, when he was completely normal, and then a photo from Tuesday and this morning. He comes out occasionally to pick at algae and to eat brine shrimp when I drop it in. I have a UV filter that I started running full time. I’m running Rox 0.8 carbon and GFO in an avast marine media reactor. The only other inhabitants are 2 clowns. The tank is 4 months old. Any ideas? There are no white spots, so I didn’t think it was ich or marine velvet. I was thinking HLLE? I can turn off the carbon and gfo if that might help. I also put some nori in the tank yesterday but he hasn’t touched it.

 

[Sea Shanty]

Yellow tangs change color during dark periods to blend into rockwork, you might see white, brown or both but that is a healthy response. Yellows are not usually vulnerable to carbon burn so do really worry about that just make sure there's something for grazing aka nori.

 

[Gcowles]

His coloration is his night time coloration due to him hiding behind the rocks, but there is definitely an irritation. I’ll see if I can upload a higher resolution picture of it. It’s only on his right side. His left side looks completely normal.