Pellets-
OSI
Kent marine reef herbivore
Spectrum, and Spectrum RX
Kent marine reef carnivore
Vitamins-
Selcon
Zoe
Zoecon(high in vitamins that tangs need, many from spirulina)
Seachem Garlic Guard
Seaweed Selects(nori)
red
green
brown(brown is best from what I have read in Aquarium fish mag)
Frozen-
plankton
brine
mysis(definitely)
bloodworms(not great, but my tangs divebomb it)
Also-tangs graze throughout the day. After I bought a Rondomatic fish feeder I noticed that my tangs color came out so vividly after a month's time. I fed them five times daily and just about every time I walk by the tank after work. Stable water conditions are a must. All the different kinds of food work too. I have had a Sohal tang for a year and 2 months and it has never even touched frozen food, only seaweed and pellets, so it is a necessity that you feed as many different kinds of food that you can afford. I have no signs of HLLE whatsoever, which I believe is highly food related. Good luck to you, the fish will look beautiful when you bring it around, it is possible do not give up hope.
I've been obsessed with tangs, and have lots of experience with them, so if you have any questions just ask away. Unfortunately I killed a lot before finding out everything I know now.