He's been in there 6 days, and initially the Yellow Tang put him in his place, but the copperband has been behaving very differently from the first time it was in the display tank. Pretty much all the fish bullied it because it wouldn't fight back, so it was too intimidated to take food, and it wasn't eating any foods anyway.
Now, after the Yellow Tang had his say, the copperband started defending himself and actually winning some fights. He's now not at the bottom of the pack and not at the bottom of the food queue.
He's eating brine shrimp confidently in the tank and he might be taking the odd bit of mysis. I've been feeding a Mussel every night, as the copperband is very active at night, when most other fish are in bed, so he gets more of the Mussel.
Previously he wouldn't even look at a Mussel, now he's pecking at it before it hits the bottom.
So hopefully that's it, fish saved.