Ampicillin (once again, lol) and some betta fix. I also reccomend you use Stess Coat (I use it as my water conditioner) because the aloe vera it contains helps regrow fins. Mthylene Blue will help as well.
For treatment use this (what I reccomend):
Get a gallon jar (keep it bare), treat it with Stress Coat. Acclimate the fish and release him/her. Add BettaFix, methylene Blue and a few drops of Ampicillin (2 or 3 drops should be fine). Change the water daily and continue for about five days. When rotting stops stop using Ampicillin and continue using the other meds. Once regrowth starts out him in a 2.5 gallon or larger tank (unless you have jar space and can provide himwith clean water).