I got the exact same symptom as yours on my copperband when I introduced him. I thought it could have been so-called "ammonia burn."
Ammonia burn is one of the problems associated with high levels of ammonia in an aquarium. The excess ammonia burns the fish’s skin, gills and fins, causing a reddish discoloration that eventually turns black as the spots start to heal. However, if the fish continue to be exposed to high levels...
animals.mom.me
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Though my ammonia level was nearly zero, my tester was very rough, like 0 and then 0.2, and I doubted it could have been somewhere between 0 and 0.2, not zero. I had been trying every food, such as clam, mysis, pellet, flake, raw, frozen, dried, etc., so heavily that time trying to let him eat. So there must have been raised ammonia, I believe.
In the same tank, all other fish were totally fine, but because my copperband was newly introduced that time, I thought he had been having some stress, which had weakened his immune system a bit.
But it was gone in about two weeks, and now he is healthy. So I believe you do not have to worry about him much as long as your water quality is ok and he shows no sign of itching.