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Clean water is the best treatment. Do large water changes daily until she starts to improve. If she looks really bad right now, try some melafix. Just remember to remove any carbon in your filter first. Good luck, and remember that clean water is the key to recovery!
What are your water conditions like? Ammonia, nitrite, nitrate and pH?
Once fin rot reaches the base of the fin it can get into the blood. Antibiotics are usually needed.
Keep the water as clean as possible. Do you I've aquarium/kosher salt? You can add some to the tank (1 flat tablespoon per 5 gallons) depending on what other fish there are or give her a salt bath (half an hour in 4 teaspoons per gallon).