I would think that if you keep medicating that disease would build a tolerance to the meds much like humans. I will be avoiding foods with copper since I don't think it is a great idea to treat for disease unless they actually have it. Would copper not build up in the fish's system to toxic levels over time? I would hope if I have a healthy system and healthy fish, I would not need to medicate or give food that has any sort of meds in it.

Am I mistaken in thinking this?

I think that applies more to antibiodics. Like eruthrimiasin (spelling??)use for killing cyno eventually it won't work if used over and over. From what I just read it is another mineral taken up buy the fish from natural seawater which dose contain small amounts of copper. Copper kills the skin and gill parasites. I can only guess why having copper in a fish would be beneficial. Maybe the fish with high levels of copper are not so tasty to a skin or gill parasite?? Interesting though I would be interested to find the real answer.I might just email the food company. Spectum??? anyone I threw mine out everyone seems to carry it.