Previouslly i have used Melafix and Pimafix and this has combatted it well. But from my assumption there has to be an underlying issue for finrot. Bad water quality etc etc.
What are your tanks details? ph, ammonia other readings etc.
right now i have him in a one gallon bowl but i think it developed from stress of my sister cleaning it. Im upgrading him to atleast a 5 gallon filtered tank later this week