The salt is a great idea for this!!! I didnt know about the Epsom salt treatment @Colu suggested. I had a Swordtail female that had this happen. Was the first time experiencing it. I didnt know what to do so I reached out to a friend and they suggested salt. I put the fish in my QT tank...a 5 gallon....I used aquarium Salt...2x TBS in 5 gallon of water. Put fish in and within 2 week all was good. The fish has since been reintroduced to the others. Come to find out that the problem came from giving birth. She must have had a rough go of it and caused a problem.
In fact when I was going to remove her I found a lone fry in the QT tank with her. Go figure.