Hey Mo-Mo
The temp fluctuation sounds like it could be the problem. Is that just from the water changes or is the temp fluctuation a normal event? Tangs prefer stable water, so the less your PH, Salinity, and temp fluctuate the better of your fish will be.
I don't believe high Nitrates would stress a Tang all that much. Perhaps in combination with a few other annoyances but not just nitrates.
Do you have live rock? Tangs love to peck at algae growing on the rocks.
For food I believe in variety, especially with Tangs. I know it's a pain with just one fish but I would suggest a daily supplement of Nori or some other unseasoned raw marine algae. Formula I is an excellent frozen food but it's packed with nutrition... so tailor the amount you feed to the size of the Tang, don't feed a whole cube. Formula II is also good if you can't find a good source of dried algae.
When feeding dried algae use a clip or attach it to a rock so it doesn't blow around. You'll also need to be careful to feed only as much as the Tang will eat.
I believe that will help a lot.
