Novice suggestion: koi fish's fin root is hyperemia, which is suspected to be trauma.

1, the body has inflammation, which is more serious. It is recommended to dilute furacilin for two days before taking oxytetracycline tablets.

2, internal bleeding. Difficult, it will often lead to enteritis, such as sudden change of water temperature (maybe you didn't consider the temperature difference between new water and old water when changing water)

It is caused by enteritis. There is a small bottle of syrup called "enteritis spirit".