Yellow Tang Died in QT. Why?

[SaltyMember]

I am trying to figure out why my Yellow Tang died at week 5 in QT. I found him dead yesterday. He showed no visible signs of disease. The QT tank was well cycled and I fed lightly once per day and kept a small amount of nori on a clip, so it wasn't an ammonia spike. He had gone through 1 week of prophalactic PraziPro treatment which he tolerated well. This was removed with carbon and a 50% water change. He was in his second week of prophalactic cupramine treatment which i do with all fish. I dosed at the recommended dosage and tested my levels once the dose was reached. A 50% water change was done just prior to cupramine dosing. I had 2 small hang on back filters and 1 small powerhead for water movement.
My water parameters yesterday were:
PH 8.0
ammonia 0
nitrate 20
SG 1.025

This tang ate pretty well (mysis, nori, flake) but one thing that concerned me is that he never got over his shyness even after 5 weeks he still would hide and only come out from cover of PVC to eat and swim when i stepped back from the tank? I am just trying to figure out what I could have done wrong or might have missed. Any input?

 

[Stuart60611]

How big was the quarantine tank, and how big was the tang? Sometime fish, like tangs, who require a lot of space get stressed out in small confines.

 

[SaltyMember]

20 long tank.....3 inch tang.