I learned the hard way the secret to getting copperbands eating is clams. Every CBB I've had for over 2 years has started on claims. The first was a blue squamosa clam, after that I learned freshwater clams work just as well.
The local super store carries trays of live clams. I would freeze the clams, then Crack them in half and elastic them to a tile. The clam/ tile was then put where the CBB was hanging out. It takes a few days but after seeing the fish eating the clam the CBB would as well. You may need a couple clams in a couple spots to make sure the CBB gets a chance to try the clams.
Now I use the clams mixed into my home made frozen preparation and the fish love it.
My current CBB was the same size as the OP's CBB when I purchased the fish. We are right about a year and it's doubled in size and actually fights the tangs for its fare share.
I have tried this with 4 CBB and have gotten the same result. I have yet to loose one. That being said IMO 50% is making sure the fish is fed and 50% the quality of fish I am getting. I always start with golf ball size fish. Never any larger.
Ha! Mine picks on my yellow tang too
Interesting you start with small ones. Bought mine as a size large for sure.