Finrot is usually from poor water quality, and you stated that your water has been good so I would say this is not due to problems of water quallity.
If your parameters have been pristine: 0ppm on ammonia and nitrite, and your nitrate levels have been 20ppm or so, and you have a regular routine of water changes to remove excessive dissolved organic compounds, and you vacuum the substrate regularly to remove decaying debris, poo, and gunk, then water quality is definitely not the issue.
Evenso, so this is probably a bacterial infection. The possibilites are several pathogens, but I think this sounds like Columnaris, from your descriptions, especially that white patch you see on your female swordtails head.
If this is Columnaris, then perhaps a new fish has introduced the bacterium into the tank. If you have added no new fish, but perhaps you added plants along with the water they were in from your lfs, that might have had the Flavobacterium columnare bacterium in it.
However, I've read scholarly papers that state Flavobacterium is likely ubquitous in most all aquariums, and there are several strains of Flavobacterium columnare with varying degrees of virulence.
So, Columnaris can be in the tank, but not causing infection while the fish have strong immune systems.
Some kind of stress weakens the immune system, or some nick or injury from an ornament, netting, rough handling, can weaken the immune system and lead to an infection.
I would step up your water change routine, get Furan 2 and Kanamycin to treat them, bring your water temperature down to 75 F, because warmer temperatures speed the growth of the bacteria, and I would add salt, plain table salt is OK, to bring the tank gradually to .3%, which is 3 level teaspoons per gallon.
There's a lot of debate about using salt, however, in studies and in documented cases, salt reduces the possibility of infection in those fish not already infected, helps to slow the spread of the bacteria on the surface of the fish, and reduces mortality in those tanks that have .3% salinity. Salinties of .1% and .2% were included in the study, but the mortality was higher in those in the tank with less than .3%.
In that same study, in vitro growth of the bacteria seemed to actually get worse with the salinity percentages, but other documented accounts, by a degreed Discus breeder and others, have shown that salt really helps defeat this disease.
I would do a big water change, having removed any carbon from the filter, then dose both meds at that time. Then slowly, over the course of three days, bring the salinity up. If you are reluctant to go to .3% try 1 or 2%, however, I would go .3%
Someone with more experience and stronger convictions regarding your case may have other suggestions, and I defer to greater knowledge and experience, but in my opinion, I think this is Columnaris and needs immediat action on your part.
Continue to do ample water changes every 24 hours, just after dosing each day, and don't forget to compensate for the salinity, adding back the proper amount of salt to the replacement water.
Make sure that the replacement water is temperature matched, and I am a strong believer in using Prime as a water conditioner.
Prime removes chlorine and chloramines, and detoxifies ammonia and nitrite in the water, giving you breathing room to do the water changes every 24 hours. Any detectable ammonia and nitrite need to be addressed, and Prime, will handle it until you do your water changes every 24 hours.
