This is a printing disease, which belongs to bacterial infection. Take out the diseased fish, apply mercuric oxytetracycline ointment to the ulcerated area, or soak it in 3ppm malachite green or methylene blue solution for 3-5 minutes, then soak it in yellow powder and 5% salt gentamicin for 2-4 hours, and apply the medicine 1-2 times a day, which can control the disease for several days. During the treatment, blow oxygen and stop eating.
Besides, this one is not koi fish, but Cao Jin. The tail is deformed by congenital defects, accompanied by the precursor of bacterial tail rot.