Depends on the cause:
1) Physical damage
2) Nutritional deficiencies
3) Poor water quality
4) Bacterial infection
What size tank?
How long has it been running?
What are the water parameters (pH, ammonia, nitrite, nitrate, anything else you have a test kit for)?
What else is in the tank?
Do you have a QT/hospital tank that you can set up and move this fish to?
If it is a bacterial infection it would be best to move the fish to a Quarantine
tank. I would just do daily water (10%-20%) changes in the QT tank for a few days to see if this helps. If not then treat with a broad spectrum antibiotic