You are working to slowly improve the water conditions.
You will need to feed the sailfin top quality spirulina disks (look to make sure the first ingredient is spirulina) or wafers soaked in selcon.
You will need to feed the regal top quality meaty food soaked in selcon. (Remember regals are meat eaters.)
IME unless a reefer has left it too late, HLLE can be reversed.
Good Luck

That study is a little swayed, obviously a tang fed nothing but sea lettuce (the seaweed equivalent of iceberg lettuce) is going to develop some pretty bad deficiencies. Commercial prepared (enriched) pellets and disks are far superior to something like that though should probably still be supplemented with a variety of foods. I've seen video of a yellow tang eating a chunk of banana.

Certainly, no big surprise there, but I think there are some important points that one can take away from that study.

1. adding seaweed to the fishes diet is most likely not going to help resolve HLLE.

2. compare the actual nutrient levels found in ones pellet/flake/disc food to those used in the study, and see how they stack up.

IMO HLLE can be reversed, without using products such as selcon, or using a shotgun approach to feeding.

Hmm, I'm sure many of us have seen that documentary 'Supersize Me' where the guy eats nothing but McD's for a month. Same sort of deal, not enough nutrients because the basis of the diet has no variety. Beef, potatoes, and salt do not make a good balanced diet :lol: and neither does flake food or any single food item, though the situation is greatly improved with the addition of other food items to vary the nutrients consumed. Selcon should not be relied on as a cure for these diseases, since the problem of a poor diet still remains.

HLLE and hole in the head are probably as reversible as metabolic bone disease in lizards and amphibians, but needs to be caught early before the effects become lethal. By the time you see physical signs, it may already be too late.

Ok, let me reword that, compare the actual nutrient levels found in your varied diet, to those found in the pellets used in that study, and see how they stack up.

One can vary a fishes diet from now until the end of time, and still be providing a substandard level of nutrients. One can also feed a single food item (that contains a wide variety of balanced nutrients) and keep fish thriving in captivity for many years.

I agree.

Hmm, maybe I should clarify what I meant... I meant that if the diet is based on a high quality pellet that it's definitely better than flake or seaweed, but that doesn't mean that an animal should be offered only pellets. I'm really not trying to argue the point here, just saying that a varied diet BASED on a high quality food item like enriched pellets or disks is a great start to keeping healthy fishes. If it sounded like I was saying that study is bunk, I didn't mean it that way.

Hope the tang improves anyhow, they are amazing fishes.