Flavobacterium columnare (flexibacter) is common too. It looks like a
fungus but is bacterial so Malachite Green won't help. In that case you
would need an antibiotic. It can be hard to tell the difference without
a microscope but true fungus is more common in cooler water while flex
is more common in warmer water. True fungus will often grow over an
open wound or sore too.

Either way, I would only treat the sick fish in a hospital tank. Both
the anti-fungal meds and the antibiotics and be pretty harsh and can
stress healthy fish.