Thanks. Here is a link:
No other fish are showing any signs. I have not added any fish, corals, or live food for months. But the tang is occasionally scratching like it has skin irritation. What could this be?
Thanks for the video. I see some HLLE on the Tang -- it's an erosion of the skin. It
is strange that no other fish shows signs of infection. Parasites 'don't care' what fish they infect/attack.
Tangs in general have a poor mucous coating, making them more susceptible to skin conditions (e.g., HLLE). Some suspected causes include (not all-encompassing) activated carbon, parasites, carbon dust, heavy metals such as copper, electrical currents, endotoxins, ozone and U.V. radiation exposure products, as well as nutrient deficiencies including vitamins A and C.
The Tang is 'fat' but is it getting the right vitamins? Do you use something like Vita-Chem? Download then read, the
Fish Nutrition document.
The 'spots' could be mucous extrusions, sometimes referred to as bacterial tuffs or viral nodules. The ones I'm most familiar with are small on Tangs, larger on some other species.
With vitamin supplements and perhaps a better diet, the fish should heal on its own. In the meantime, do read that link and treat the fish (all fish) as if it was an ill fish. Help the fish by improving its immunity and ability to heal wounds by adding supplements for an ill fish to its diet as recommended in the linked document. Click on that link, then download (and read) the document. Use especially Beta-1,3/1,6-D-Glucan found online, in health-food stores, and pharmacies. Directions for its use and quantity is provided in the link.
Hope this helps.