I had the same thing happen. several months ago, my largest black skirt started growing a lump on her side, and also a little growth on her face/chin. both areas were black in color, and both continued to grow. finally, the lump started to rupture (an abscess or tumor) and her mouth fell off on the same day. literally her jaw was just dangling for a little bit and then broke off, and her face was just open. i put her down that day. aside from the obvious physical damage, she never acted or appeared sickly. always swimming, chasing other tetras, very perky. kept eating right until her last day.