Impossible to say for sure but it is either surface skin damage (scratches) or an ulcerated wound. I am sorry to say that it sounds like the latter of the two. You need to get a local expert to see the koi to make a proper diagnosis.
Treatments are as follows:-
Scratches: Clean wound and apply antiseptic solution to area. Koi will heal best in a "clean" well filtered & heated quarantine facility.
Ulcers: Quarantine Koi, Slowly raise heat to 20-25oC, remove dead skin & scales, apply antiseptic solution to wound and inject with a course of antibiotic. Baytril is recommended.
In both cases, you will need to put the Koi out so you can work on it. If you are in any way apprehensive about doing this, consult a local vet or koi expert.
Posted on Apr 5, 2005, 6:32 PM from IP address 213.120.101.21