Hi Kristen, sorry to hear about your fish.
I sorta agree with seaham, it doesn't sound like ich. Have you read anything on Headd and Lateral Line Erosion (HLLE)? It's, unfortanetly, common in Tangs.
http://www.reefkeeping.com/issues/2005-06/sp/index.php
Your nitrates at the 15-20 range aren't high enough to cause long term issues, IMHO.
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