Yahoo Answers... *snickers*
Melafix can suffocate your fish. Your fish has to breathe. Forget that Melafix crap. Even if it helps accelerate the recovery rate of damaged fins, the concentration does prove lethal to all anabantoids.
Stick to salt. A lot of bettas are already domesticated and adapt readily to various water conditions so I do not believe that the osmotic balance becomes compromised to bettas that have adapted well to hard alkaline water so using salt is not really an issue for that. You do need to keep water well aerated as salt reduces oxygen level. While bettas can breathe from then surface, aiding their respiration at this point will help reduce their stress levels.