From what I've experienced, my advice would be to quarantine all of your fish, divided tanks and all, and watch them like a HAWK for ANY sign of illness. Sometimes the symptoms are extremely hard to spot, and you don't see them 'til it's too late... but the things to look for are lethargy, paleness, quick onset and development of fin rot, bloody fins, white, cottony spots on the mouth or body, unwillingness to eat, spitting out food, pain while eating, red and white lesions on body or face, literal "chunks" missing out of the fish... It's hell, really.
Treat immediately, but if it seems like they're too far gone, my advice would unfortunately be to put them down. They will develop severe dropsy and die within two weeks after the onset of an internal infection. I made the mistake of trying to save them til the last day, and they suffered way too much. If I could do it again, I would have put my girls to sleep. I was hurting and stressed to the point of physical problems, and they were hurting too. It's just not something you want to draw out. Good luck, sjones, hopefully your strain isn't as horrible as mine.