In my experience, this is not particularly contagious, so personally I don’t think isolating the fish is strictly necessary. Also, it is early, so it does not need to be considered fatal. And also, whatever it is, it is in the tank.
The approach I would take is to treat for the infections, while at the same time address the possible root causes, which often are a combination of multiple factors. Specifically, consider water quality (regular levels of nitrate, and amount of organics); consider parameters (temp, hardness, pH); consider diet and increase variety and quality if found suspect; and then, consider any other potential sources of stress, both on the individual fish, but also generally. Identifying manners of improving things may be effective, perhaps more so than medicines.
This may or not work but it is what I would do and what I one needs to do.
Good luck!
