What's the water quality like (ammonia, nitrite, nitrate)?
Fin rot is normally caused by poor water quality that damages the skin and allows bad bacteria into the tissue. Cleaning tank conditions up and adding salt usually fixes the problem.
You can try doing a 75% water change and gravel cleaning the substrate every day for a week or two and see if it helps.
Make sure any new water is free of chlorine/ chloramine before it's added to the tank.
You can add rock salt at a dose of 1-2 heaped tablespoons per 20 litres (5 gallons) of water and keep it in there for 2 weeks.
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Make sure you don't have anything sharp in the tank because it can cut or tear the fins.