Can we compare notes on a few things?
20 gallon QT that has been fully cycled for over 35 days.
Aquaclear 50 HOB filter with BioMax ceramic media that has been soaking in my sump for 30+ days; foam filter pad soaked the same.
Dosed tank with Seachem Stability as directed on the bottle.
*** doing the two steps above, I've never seen ammonia test in the QT. Only change water about once a week.
Powerhead aimed up a surface for better O2 exchange
Dosed Copper Power in small amounts AM, lunch, PM over 5 days to reach therapeutic level
Fish came in at salinity of 0.019 from Live Aquaria.
Kept it at 0.020 until the copper treatment was done.
Food:
@jsker turned me on to making fish "smoothies" to give the fish a wide variety of proteins, fats, and vitamins. Several different frozen fish cubes one each: chopped clams, mysis, brine, cyclops, seaweed, reef mix, etc. Allow to "slump" in a tupperware in the fridge. Pour off excess liquid. Add liquid vitamins a couple of drops like Selcon, Zoecon, etc. The mix stays good for several days in the fridge. My fish love it.
Have used this process for puffers, wrasses, anthias, etc. that are all usually called "copper sensitive." Have not lost a single fish. I think it's Jsker's smoothie idea tell you the truth that's making the difference, and of course
@HotRocks awesome research on the Hanna. I and my fish owe both of these guys big time.
Hope this helps a bit. I just love my Blue Toby. Sad for you to lose two of them. Don't give up

they are such cool fish.