i have seen something like this before. red streaks lining the scales, and sometimes the area is not swollen, sometimes it is. i believe its bacterial in nature. possibly septicemia or if you're lucky something more localized. in any case, whenever i see this, i'd always use salt and antibiotics, such as kanamycin or tetracycline, because this infection could act fast and weaker/slower meds often don't do any good.