I am thinking that the info I have pasted below fits with the symptoms, but I feel the need to make clear that I've done a google job here & I know nothing about fish ailments so take what I'm saying with a grain of salt. Hoping it will help you problem solve as you are the one observing the fish first hand:
"Hole-in-the-head disease is commonly associated with tangs (and angelfishes), but relatively little is known about what it is, what causes it and how to effectively treat it....
Hole-in-the-head disease is believed to be caused by a flagellated parasite that occurs commonly in the gastrointestinal tracts of health marine fish. When populations of this parasite grow too large, infested fish will lose their coloration and their appetite. They will act abnormal (e.g., a tang sulking in a dark corner of the tank) and their fecal matter may become white and slimy in appearance. Pitting of the flesh on the fish’s head is also sometimes observed (hence the name), but the overgrowth of these parasites usually occurs in the intestinal tract.
If you suspect hole-in-the-head disease, it is recommended to remove the fish to a quarantine tank where it can be nursed back to health. In the quarantine tank,
metronidazole can be added to the water at a concentration recommended by the drug manufacturer. Seachem makes a widely available product containing metronidazole called
AquaZole. There are other options including the human drug
Flagyl.
http://www.bluezooaquatics.com/resources.asp?show=378
Also my book on Marine Protozoa wasn't very helpful, but an interesting tidbit I read was that when bacterial protozoa colonise and reproduce in large numbers on a fish skin & gills, it causes increased respiration or panting.
Does anybody think there could be a chance that if it is an internal parasite it may run its course and get past it in time on his own? If there was a fish in my tank I would back to survive something it would be this one. Apart from the immediate problem he is a big healthy strong fish living in quality water parameters.
I do think this is
possible IF you can get the tang to eat & get the animals digestive process operating properly again - choosing this option is a gamble with no way to know how it will turn out because you don't know how numerous any parasites are right now, if thats the problem. Also I'm not sure how long a fish can safely go without food before it becomes weak. However, First you want to clear or flush out the gut to reduce inflammation and push through any food sitting in there that can cause gassy buildup and/or blockages (imagine last weeks lunch still stewing in your stomach/intestines & the gas acts like a cork). If you can manage this, you will see all of the nasty stuff coming out, then it will stop and appetite should return. You won't see any poop for a while after eating resumes, and then when you do, it should look normal. Thats how you know normal digestion has resumed and the fish is getting the most nutrition out of its food. That gives its body a chance to operate properly and fight off ailments. I go through this process all of the time with wildlife rescue as many of them can't digest food properly due to dehydration, so I know how and what to use to do it for certain animals, but have never done this for a fish. I can't even begin to think how to go about it because the tang isn't eating, the only method of treatment that leaves you is some kind of soak. I have two ideas - but they really are just me trying to apply the same basic idea as what I have been taught (in a backyard first aid kinda way!) to fish - so I have no idea what exactly it is that they will achieve, just that they are harmless options.
The first is food: I was thinking offering an oily food soaked in garlic might entice it to eat a little and the lubrication will help anything in the gut to be pushed through. You could add a
tiny drop of vegetable oil to the food to make it oily.
The second is hyposalinity treatment: In QT only, gradually lowering the salt gravity to 1.009 as suggested in the link below. You need to follow the instructions of how to do it carefully though as if you don't the fish could be killed:
http://www.aquariacentral.com/forums/showthread.php?144242-A-general-guide-to-Hyposalinity
My thinking is mainly that by doing this you are expanding the cells in the tang's body - so it could be the equivalent to 'hydrating' it while it is sick and, as it says in the article, it will also reduce stress which is crucial IMO.
If I am wrong about that, at best, it will
possibly break the life cycle of anything parasitic.
