What are you using to treat for hole in the head? Is it safe for scaleless fish (I'm guessing its an antibiotic and they usually are, but better to check. If it is, it may muck up your biological filter...be careful)?
Have there been any changes in the tank? New substrate, decor, tankmates? When you say perfect levels, what are the numbers exactly?
Plecs change color for a number of reasons; camoflauge, maturity, stress, environment. Hard to guess whats going on with your guy without more info.
aka Cycling Guru and the Ich Slayer
*glares at Terry and QTOFFER*
Card carrying member of FTAS & GCAS.