I think that the general idea about fin rot is that it is caused by bacteria that are very often present and that only get a chance to cause harm when a fish is weakened, for instance due to poor water quality. So the cause would be bacteria that are generalists and opportunists. I start to doupt that this is always the case and would like to have your opinion(s). I doupt it because of the following observations:
1. a few years ago I observed in a pet shop that fish of really many species (almost any they had) suffered from fin rot, despite the fact that they have several hundreds of aquariums. I find it hard to believe that the water quality was poor in all aquaria. At that time I lived in the same region and used the same tap water, without problems. I feel that the employees spread fin rot with wet hands from one aquarium to the next, and they (unfortunatelly) only recognised advanced stages. Of course I informed the shop owner, but he was not very willing to listen. Regardless, this gives me the impression of a very strongly pathogenic cause, which is not species specific and also independent of "weakened fish".
2. I once had some firemouths which sufered from fin rot, while juvenile convicts and mosquito fish in the same aquarium did not get infected. I am aware that firemouths are the more sensitive species of these three: this is an observation in favour if the "weakened immune system idea.
3. I recently had Ancistrus with fin rot. Convicts, but also swordtails in the same aquarium did ot get infected. This puzzles me because the swordtails quite frequently obtained scratches/torn fins and worse from the convicts: there would be a very big chance that the swordtails would get infected, however, they really did not. Fin rot entered the aquarium because I bought an adult Ancistrus with some slight damages in the tail fin. These damages later turned out to be beginning fin rot. This observation seems to indicate a pathogenic, species specific cause. (BTW: these same Ancistrus were breeding, despite the beginning fin rot - I don't think their condition was very bad.)
My idea is that there may be a variety of bacteria causing fin rot, some highly infective and dangerous for all fish (healthy or not), some species specific and some only occuring on really weakened fish. Wonder about other observations ...