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Mike is spot on here. Your fish's problem is about it's environment and not about a disease 'on' the fish. The bacteria is attacking your fish now because your pond is an ideal home for them , for some reason..
There are many causes for the pond environment to go bad- Mike mentioned decay from waste and leaves. It can also be from over feeding, poor water circulation, dying algae, too many fish and anything elese that might have organic material rotting on the pond floor or within the water column ( so you may not actually see waste) .
These symptoms can also be from ammonia burns or excess nitrite or even excess nitrate. Also parasites, especally trichodina and chilodonella can be a contributing factor as they also love the same water conditions that these bacteria do.
One of the challenges you face now is winter temperature. Do you live in a cold climate? If so you may have to move your koi inside to treat- this is very tricky as in the inside conditions must be better than the outside conditions which is difficult to achieve on a moments notice. So think this decision thru carefully.
First things to check
1) ammonia
2) nitrite
3) nitrate
4) pH
5) check them for parasites using a scaping of slime coat and an exam
6) check your stocking rates- one fish per 250 US gallons ( 75 gallons for small fish)
7) stop feeding your fish for 5 days.
8) remove any debris in the pond or on the floor- especially dead algae and leaves
9) do a 25% water change weekly- or even biweekly! Watch out for chlorine/chloramines
10) add aeration to mix water levels
11) make sure your circulation is at least once every two hours ( that's ALL the pond water) thru the filters.
12) add a moderate amount of salt
13) See if you can get your hands on some potassium permanganate and some nitrofurazone powder.
14) find an advanced hobbyist/koi club in your area and see if you can get some hands on help.
15) have those same individuals review your filtration/ pond design, maintenance and feeding technique.
Best of luck, JR
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