I've had a Kole for around 6 months.
She did terribly in QT, so much so I had to take her out and get her in the main tank after 3 days. Just wasn't happen. Hid the whole time and didn't take to food. She was from Diver's Den though, so I was not too worried about disease when it came to moving her into the main tank.
Once in the DT she started grazing the rock work within 10 seconds and hasn't stopped except to sleep. She's constantly chomping on the rock work, the glass, even the gravel and occasionally turbo snail shells
At first she did not go for algae sheets at all. Eventually though she took a liking to them, but seems to prefer
Julian Sprung Purple above anything else. She generally doesn't dig the green Ocean Nutrition (and now i have 2 packages)...but I
have seen her eat it. The purple she devours though. I haven't tried the red yet. I do sheets on a clip 2-3 times a week - no soaking or anything first. I just tear off a sheet, clip it, and shove it on my glass. Within 2 minutes she's attacking it like i do a rack of bbq ribs.
I also offer
http://www.bulkreefsupply.com/catalog/product/view/id/1527/ (mixed) with every other normal feeding or so, which, speaking of, ... I feed a variety of dried foods that all my fish eat, daily. Formula 1 and 2 flakes, Prime Reef flakes, New Life Spectrum pellets (small, or thera a), the above sea veggies flakes, sometimes OSI mix or Spirulina flakes..
I also (3 - 4 times a week ) feed frozen mysis, marine cuisine, omnivore blend, rods, roggers ... something else i can't recall. (not all at once obviously)
Occasionally i add 1 - 2 drops of Kent garlic extract or
elos vitamin
My theory on feeding my fish is that i offer a smorgesborg of different foods every day, so they all get something that they want, given that every species (and in some cases even individual fish) like different things.
If i were you i'd try formula 2 flakes and something else meaty (my fish seem to like prime reef) at the same time, so it's all mixed up (add 1-2 drops of garlic too if you want - i kinda just drop it on the handful of flakes before i dump them in. I'd also get those purple julian sprung sheets and offer a 3x3 or so sized sheet a few times a week.
Remember this too - Tangs are omnivores, not herbivores. They DO eat meaty stuff. That gets overlooked a lot.