Thank you for confirming that.

If there isn't a specific reason for not using antibiotics (ie. organic set-up, etc), and you wish to try treating this fish then I would strongly advise that you start antibiotic therapy in this fish as soon as possible. Bacterial sepsis needs to be treated aggressively with antibiotics to clear the bacteria and give any chance of survival.

If this koi was human, he would be hospitalized and receiving IV antibiotics (because it is that serious a condition). Now hitting a vein in a fish is not something we do often, but this doesn't mean we don't want to treat this bacteria as best we can. In this situation, you will want to use an antibiotic like oxytetracycline or doxycycline. This can be administered by treating the water, adding it to the feed, or using medicated feed preparations. Typical treatment length is 10 days (longer if not totally back to himself).

Do remember that if you do use these antibiotics, protect your biofilter and monitor water parameters closely. You may also need to do partial water changes between doses. (how much/how often will depend on the water parameter findings).

Overall, he is a sick fish and this sounds to be quite advanced at this stage. Therefore, we do have to appreciate that his prognosis is guarded. Still I would say he deserves a chance. Since he is so advanced in his illness, you may wish to treat for a few days and consider euthanising him if he isn't showing signs of improvement (to give him a chance but not prolong his suffering if he is too far advanced).