[Myka]

Wow, that's a lot of dead fish. Have you considered switching up your quarantine and treatment methods? I would suggest a bigger QT for Tangs unless they were ridiculously small. Also, I've found copper to be a harsh treatment and too much monitoring and messing around. I've used Chloroquine phosphate with better results. Another thought, anything I QT Tangs or any other fish known to be Ich magnets I always start acclimating them to hyposalinity within the first few days. Hyposalinity isn't a good treatment for Ich, but it is a great preventative. For Tangs like Powder Blues and Achilles you could do a FW w/ methylene blue or Formalin dip as soon as you get them while you're acclimating which will at least knock off parasites.

One last thought, imo an Achilles Tang does not belong in a 120-gallon tank. They are particularly active Tangs and really do better in much larger aquariums.

Ok, one more thought...copper treatments often mess up the results of certain ammonia test kits. I do wonder if ammonia is your culprit or maybe there is an issue with that Rubbermaid tub.