Matt...If you did not see any signs of aggression on the tang (nipped fins, other trauma), any signs of disease/infection/parasites and all parameters are okay I would look to the diet of the purple tang. When my purple tang died I actually learned a lot about nutrition (thanks to Steve). What are you currently feeding, how much, how often?
Some people are like slinkies...they serve no real purpose yet can still bring a smile to your face when you push them down the stairs!:p
Have a great day! Brian