What are the water parameters?
Ammonia, nitrite, nitrate, pH, kh
Water quality is most critical aspect.
The link in my signature is a big help with tank care.
Having excellent water quality allows the fish's body to work on healing rather than dealing with stress and getting more sick.
What size is the tank, how long gas it been running?
Are you familiar with the nitrification cycle? If not info is in the Aquarium Advice article in my signature link.
Any fish harassing it, any other fish ill?