Endgame is right, bettafix and melafix are the same thing. Bettafix is simply a diluted form of the product that makes dosing the generally smaller betta tanks alot easier. With either product, the final concentration of the active ingredient, melaleuca extract, is the same.
There doesn't seem to be a consensus on whether melaleuca extract is irritating to the labyrinth organ od anabantoid fish. Some people have had bad experiences, while others like myself have had good results.
In other words, use this product in betta tanks with caution and promptly remove it (charcoal works great) if the fish shows any signs of distress.