The major drawback of copper is that it does not treat Brook....but if you aren't seeing mucus sheets coming off the fish, I don't think it is Brook.
As for the eyes - tough call. Without doing a sensitivity study, there is no way to know if a given antibiotic will work or not, we just go with ones that have a good track record in similar cases. It could be this one isn't going to respond to Erythromycin. However, jumping from one antibiotic to another is often a mistake, they take 5 to 7 days to work. At this point, I would continue the Erythromycin, but consider adding a second antibiotic like perhaps Maracyn 2 or Neomycin.
Jay