I actually meant bacterial, that was my slip, sorry. But the Melafix did without question negatively impact the fish. And they did not die because I have the ability to recognize negative signs in my fish and I try to deal with it, and here they were being poisoned by Melafix.
Characins in particular are highly sensitive to any and all chemicals/medications/substances. It is because of their heightened chemical response system. My friend was not so lucky and he killed his fish with Melafix.
Understand fish physiology is key to success. Every substance added to the water gets inside the fish, into the bloodstream and then the internal organs. And they affect the gills, which is why respiration is usually a key indicator of trouble. Though that trouble can be from many very different sources. It is sensible to consider that fish in nature do not all die off from these issues, which are certainly present; if they did the way aquarists assume they will if not treated with this or that, there would be no freshwater fish left alive. Let the fish heal themselves. It is after all only our fault as the aquarist that they have the problem/disease/bacteria/fungus in the first place.