Fin rot is contagious, and if it's untreated the fish can certainly die. I just saw the post about your angel, I'm sorry you lost him.
If you have test kits for NH3, NO2 and NO3, check the levels. Fin rot can be a result of stress, and one of the major causes of stress is poor water quality.
Frequent, large (~50%) water changes won't hurt and will probably help a lot. Keeping the tank clean will help your fish heal, and if there are elevated ammonia/nitrite/nitrate levels in the tank this will bring them down as well.