Ouch, its progressing? You still adding metronidazole? If not get some, it realllly helps with HITH and stuff correlated in that area.
Metronidazole from kvvet.com. 100 250mg tabs for $13.99. That's enough to treat 1000gals.
I doubt its a type of parasite, although puttin a UV Sterilizer might not be a bad idea for the long run just in case something else comes up. Your not mixing chemicals are you? Add some charchol and remove all meds. from the tank for three days , then start the treatment again. First treat with Melafix AND Pimafix for 5 days. The Melafix will help fight the ulcer and help it grow back as well as any bacterial infection, while the PimaFix will deal with any fungus that might be present. Also while you are treatin for it add 1 tablespoon epson salt per 5 gal to the tank. After five days, if there is no improvement, remove the chemicals with charcoal, wait three days, and start treating with Metronidazole. I wouldn't treat with it while you are using the Melafix and Pimafix as theres no tellin what the medications will do to each other. Hopefully that will work out for yaz, keep your fingers crossed.