Tang problem

[Jbdrywallking]

Help! Have a 28 gallon qt tank fully cycled with 2 HOB filters. Water parameters are 8.2 ph , 15 nitrates. Recently ran 2 tangs through quarantine Kole and a Purple following Humble fish protecal to the T. Treating with Copper power, Metroplex, then using cuprisorb to remove all copper and finishing up with water changes. I followed up with Prazipro treatment followed by another water change. Both fish looked great eating all types of food. 2 weeks after all treatments the Kole Tang slowed down eating, started moving lethargic and couple of days later died. 4 weeks later same process for the Purple Tang. Fustrated by this. Any thoughts of what may of happened. I have a Hippo Tang in my display tank for 3 years now no issue. Any help would be appreciated.

 

[LAReefer4Life]

What size is your display tank and how many fish? Did you notice any aggression issues? Were there any other strange behavior exhibited by the tangs before their demise?