I would not use Melafix. It contains oils which may coat the breathing labyrinth, making it difficult for them to breathe. Until more research is done, I would try to avoid it, if possible.
I would increase water changes. Does the tank have a filter? For a healthy fish in a 6 gal tank, I'd recommend doing one 50% water change per week, with an occasional vacuuming. But, since he's fighting persistent fin rot:
I would do two 50% water changes per week.
Frequent water changes will remove toxins and bacteria. The Fungus Clear probably maintained his condition by killing off bacteria. Doing more frequent water changes will help control the bacteria levels in the tank.
(Aquarium salt contains sodium, which stresses the kidneys, and can lead to fluid retention, bloating, constipation and buoyancy issues. So the general rule is to never use it in high dosages, and to never use it for more than 10 days.)
If the more frequent water changes don't seem to help, I would try a different gram negative antibiotic. But try the water changes first.
Last edited by LittleBlueFishlets; 05-30-2013 at 11:53 AM.