Sorry to see. Most likely internal parasites. Were they eating normal? With your recent 12 fishes dying I think you should reevaluate your QT process and the way you are introducing fishes to your display tank to reduce aggression.
What was the RODI at .07 mean?
Do you use an acclimation box?
Do you QT with anything else besides copper?
If you really plan on QT everything and I mean everything (fish, corals, inverts etc.) maybe catch out all your fishes and go fallow on your display tank as a reset.
The pbt was in dt for 7 months or so. It shows sign of ich so I placed it in qt for a month. It was fine. Put back into dt, got ich again. Put back to qt for 6-7 weeks. Copper and hpo. It was fine so Put back to dt. Next day show sign of ich. Took it to hospital tank (no copper no hypo) died next day.
It infected other tangs.
However other fish died due to harassment. They look beat up.
Today removed yellow tang and checker warsse (no sign of damage fin? into 75g. Only see white spots. No copper yet.
Caught the labouse wrasse and watanabe angel. They look beat up. Put them in sump.
Caught one bastid black snowflake (healthy) trying to catch the other. I think these two are the bullies or at least one of them. Because when I got them, owner had a divider inside tank to protect the wyoming clown.