sailfin with HHLE

[tlo]

I have a red sea sailfin ~ 8" that has developed HHLE on one side of her head.

She gets a sheet of nori a day (shared with1 other tang), frozen food supplemented with vit C, and occasionally some pellets.

water params
-sg=1025
-nitrite =2
-ammonia=0
-temp = 26C/79F
-ph = 7.8 (low, but it is steady at this since I set the tank up 2 years ago)

no stray voltage

Any other ideas on what could be causing this? It seems to be getting worse instead of better. She is queen of the tank, so I dont believe it is stress.