I believe that "stress" is one of the major contributing factors for the onset of fin-rot. Maybe the two affected bass needed Valium while the others were able to tough it out.
Seriously; you indicated that the pond was overstocked in March of this year. It is very possible that the affected fish may have stressed during transport, or even after they were stocked in a high-density environment.
I'm not sure what treatment(s) - if any - should be considered. Non-iodized salt? Potassium permanganate? Reducing the population-density?
I would suggest consulting a fisheries pathologist - preferably with an affected fish-specimen in your possession. If it turns out to be something else, fine. But, if it is fin-rot, you don't want to ignore the possibility that it might spread amongst the remaining bass population.
