We're going to need a lot more information than that..
1. Water parameters. (ammonia, nitrate, nitrite, PH, temp', Hardness etc) 2. A full description of the fishes symptoms. 3. How often you do water changes and how much. 4. Any chemicals and treatments you add to the water. 5. What tank mates are in the tank. 6. Tank size. 7. Finally Have you recently added any new fish?
A picture could also help.
*Edit* Is this the same fish in THIS topic, or a different one? Just checking!