I do not use copper in my tanks but a colleague does. He used copper in his classoom tank because of suspected ich. Evidently his clownfish has been swimming abnormally, on his side, at the surface. I advised him to remove the fish into clean (non-copper) water and to keep an eye on it.
Like you, I think that a copper OD would result in death of the fish. There may be something else at play here but I thought maybe it could be the copper...