Here are some steps you can follow to treat fin rot in betta fish:

1. Perform a 60% water change of your betta fish tank and add conditioned water.
2. Clean the substrate using a gravel siphon to eliminate solid debris and other waste.
3. Maintain a stable water temperature between 75-78 °F.
4. Remove the carbon from the filter.
5. Add aquarium salt to the tank water (1 teaspoon per gallon) and dissolve it completely.
6. Add an antibacterial medication like Erythromycin or Tetracycline to the tank water according to the instructions on the package.
7. Monitor your betta fish closely for any signs of improvement.

Remember that recovery can take a while depending on the damage . I hope this helps!