Unfortunately, these initial signs are often missed and the infection is not noticed until a larger open ulcer appears. An ulcer is a break in the skin, often circular but not always, which is usually inflamed, looking very red and angry. Because the appearance of a infection is not always noticed until fairly advanced, koi-keepers tend to think the 'sudden' appearance of an open wound is due to the fish cutting itself on a pipe or rocks, They may leave the wound to get better but in the case of a bacterial infection it simply gets worse. Fish do occasionally cut themselves but it is rare and the appearance of any open wound should be taken seriously, particularly if there are signs of inflammation. These open lesions usually become infected with other bacteria and fungi which obscure the underlying cause of the ulcer.
Fins are another main site for infections.
The initial signs are at the edges of the fins. In some instances the resulting 'fin-rot' appears as a discrete bite shape or a noticeable reddening. All fins are susceptible to infection, and this is usually stress related. Infections to the lower anal and ventral fins are relatively common so inspect the 'undercarriage' during your regular health checks. Again, it is rare for this type of damage to occur from fish biting each other or knocking against walls and pipes.