Hi,
Could you please take some pics of the wound? Is the fish isolated? What are the water parameters. Please post the exact results. What fish is this? Tank size (including hospital tank) and tankmates? Isolate the fish and stop using Melafix. How much salt did you use so far? Try three teaspoons per gallon to keep secondary infections at bay. In the meantime, get a Q-tip and apply iodine solution on the wound. If this is ulcerated, we may need a stronger treatment but we'll need pics first.