These black spots are very tiny and don't look like pox scabs at this stage to me. It could be mosquito bites or peck marks.
This page has a comparison between canker, wet pox and thrush. The copper should work against thrush, which is a yeast, but it doesn't look like thrush to me. The metronidazole should work against canker and if it goes away they can still carry it but there shouldn't be any new infections and lesions. You can let the feeders dry out, the canker parasite can't survive long out of the body. For wet pox there isn't much you can do afaik, but it can take months to get rid of it.
Now all these diseases can look a lot like other bacterial or fungal infections. So you might need an antifungal instead of antibiotics. To make matters worse, it could secondary infections too.
Is there any chance you can see a vet for a diagnosis?
This page has a comparison between canker, wet pox and thrush. The copper should work against thrush, which is a yeast, but it doesn't look like thrush to me. The metronidazole should work against canker and if it goes away they can still carry it but there shouldn't be any new infections and lesions. You can let the feeders dry out, the canker parasite can't survive long out of the body. For wet pox there isn't much you can do afaik, but it can take months to get rid of it.
Now all these diseases can look a lot like other bacterial or fungal infections. So you might need an antifungal instead of antibiotics. To make matters worse, it could secondary infections too.
Is there any chance you can see a vet for a diagnosis?