Don't add any more salt as salt oes not evaporate. There is an all natural product called PIMAFIX that should do the. treat the fish in a 30 minute bath 3-4 times per week for about 10 days, and you should see a drastic improvement. Don't treat the entire tank as some fish (especially tetras) can be sensitive to PIMAFIX. Treat in a 1 gallon container for about 30 minutes and return to the tank. Increase your water changes and check your filter media for age and channeling. Fin rot is a bacterial problem in the water manifested on the fish. Additionally you can dose formalin 36% every other day for 10 days the any worms, cysts, or parasites should be good and dead! If the fish has open sores an antibiotic of sorts should be beneficial. A natural remedy is a healthy dose on Indian Almond leaves which has natural anti bacterial properties well as softening the water. Filtering your water through peat pellets is also another method of softening the water, which your fish will appreciate. AS a last resort, you can net the fish and treat the affected area with hydrogen peroxide. Treat only damaged area, and avoid getting it in the eyes or near the gills. 2 minutes of this treatment 3-4 times per week will be quite efficient. Finally, you can take a crushed vitamin c tablet and add to Hydrogen peroxide bath to kill both bacteria as well as any parasites. Treated metro flakes is also a good choice to kill any internal bugs. Time is not on your side, so you will need to hurry. These simple steps along with increasing your O2 supply should end this difficulty you are having.
Good Luck
D