so about 3 months ago I put a bunch of my fish from my 200 gallon main display into quarantine tanks due to a nasty case of ich, I put copper in the tank and just let it do its thing one of the fish I put in these tanks was a purple tank that was super health he stayed health for the whole time he was in this tank until 2 nights ago I started realizing that he had a massive loss of his appetite and he was very lethargic and he was constantly breathing heavily and he seemed to have some weird white film in his gill I thought it was just stress so I took all the water out rinsed the whole qt setup and did a 50% water change and replaced the copper but by the next morning he was gone and he was very stiff to the touch, can anyone maybe help me figure out what happend and how to prevent/treat it in the future.