Hi! Im sorry no one has come to your aid. I am honestly not positive what this is but it looks nasty. If it had just appeared overnight, I would guess an injury but you mentioned that it has progressed from a white spot so I am not positive what it is. Have you googled HITH? This would be a possible suspicion. I would start by adding aq salt (1tbsp per 5g) and consider treating with metronizadole. You will need to remove the melafix from the water first. Seachem makes a metro treatment (try American Aquarium Products or DrsFoster & Smith) or try National Fish Pharmceuticals for their metro. This is my best guess here because I am not positive what is going on. Good luck!