You'll probably get different ideas here Steve, but here's what I do:
~Heavy water change and a good cleaning of the substrate.
~Remove carbon, if present, to avoid filtering out medication.
~Treat the whole tank with metronidazole according to dosage on product.
~Feed NLS Thera A exclusively.
In my experience the fish that are showing severe symptoms almost never recover. If you have a quarantine tank, move them and treat them the same way....but definitely treat your main tank. If anything, that's more important...to save anything else from getting to the point of no return.