Well here comes the flames. While you wouldn't want an emaciated fish, an obese fish is not healthy. It was to your credit that you maintained that species and got it to eat so well, but in that picture it is simply fat and no longer in prime condition.
All tang species have a high metabolism and can cover tremendous areas of the reef grazing for their food. However in our tiny boxes of water they seldom get the exercise needed to burn off the high protein foods and especially that which contains elevated levels of HUFA's.
Unlike humans, fish grow their fat towards their innards, so when you see them as plump, it is really much fatter than what appears. That fat will begin to crush the organs and cause health issues.
I'm not gonna flame. Just respectfully disagree.

I've never seen an obese fish. I assume it may be possible, but it would be very very rare. Fish do store energy as fat, as you explained, in the upper portion of their internal organ cavity, but they also store vast amounts of energy in muscle. Right now you can see my tangs spine. Eventually, I'll fix this and you won't be able to see the spine. This will be due to muscle mass. Not the accumulation of fat. I can't see the stores of fat, inside the organ cavity, getting large enough to cause problems for internal organs. Do you have a link to show that this takes place?
Tangs, and most bony fish, have to have very elastic internal organ cavities. They often live a life of feast or famine. When food is plentiful, they must take advantage of it. To do so they must be able to take in large quantities of food at one time. They must be able to do this without causing damage to internal organs. Tangs also ingest sand or gravel of different sizes, depending on what's available in the environment. If it's larger grain size, they must be able to ingest it without causing damage to internal organs.
I've never seen, or heard of, a health complication from a fish getting to much to eat. I have seen many complications, illnesses, and death that stem from malnutrition, though.
I have seen no evidence, and have no reason to believe, that a well fed, and thick, tang is an unhealthy tang.