Well, just try to stay positive and treat as soon as you can. I've already listed some meds in my other post, Jungle Fungus Eliminator and methylene blue. The Jungle brand also makes a Fungus Clear product, but I haven't tried it. My meth. blue is made by Kordon, but there may be other brands at the
lfs. If you don't see those brands, ask or look around for another brand of fungus medicine.
I would try the Jungle Fungus Eliminator first. It also has an antibiotic to guard against secondary infections. But one time I had a stubborn case and the meth. blue worked for me.
I keep those fungus meds, a few antibiotics, and a parasite med on hand just in case. I get the smallest bottle/cannister of the med that I can, because some of them expire, and some of them you may end up not using. But I found that it's easier for me to keep a basic supply on hand. If the fish is not resonding to one of the meds, I can then go out and get another one, but it does ease my mind to have a few meds on hand to start off with.
I would just
QT the affected fish. Keep an eye on the others, of course, but my reasoning for just treating the sick ones is that fungal spores are everywhere, in the water and in the air. The sick fish succumbed to it, but the healthy ones did not, and hopefully they won't. When my fish got fungus, I think it was from an injury. I think he may have scraped himself on a rock. Is there anything that could be sharp to the fish in your tank? I wouldn't treat the fish that are not showing any signs of illness. You also don't want to expose fish to medicine when you don't have to. Good luck!