Koi fish has bloodshot fish scales and red and white fish tails. Generally, it is viral hemorrhagic disease and bacterial tail rot. My experience in this case is: first swim with potassium permanganate water for 30 minutes; Then swim in salt water for 20 minutes; Then swim in gentamicin or penicillin for 30 minutes. (Oxygen pump must be added in the above process) Stop eating during taking medicine. The above medicines are sold in fish markets and Taobao fish shops.