Koi Bacterial Infections of Koi and Goldfish
...are ALWAYS caused by some form of stress, whether it's seasonal chilling in winter, parasitism, poor nutrition, or other environmental factors like handling and transport. There is little doubt that the average hobbyist would be best served to combine Medi-Koi medicated koifood and antibacterial bathtogether to recover their koi. Not only are the treatments effective, they are also cost effective.
What should I know about a bacterial infection?