Is that a wound on her side? I see what looks like a stab wound.

Sorry to hear your fish is not doing well. I cant make out the pic. A wound could be an injury or sore caused by disease.... which can be caused by fish getting stressed or weakened by poor water quality or overcrowding. Re: water quality, there are other factors that the tests wont catch - e.g. oxygen levels, dissolved organic compounds (foamy looking water). How many fish in how many gallons and what is your filtration, pump size etc. ?

Nooo its not a wound its a bubble from the surface sorry

I don't know of any standard tests that look for dissolved oxygen levels. What are you doing to keep oxygen up in this pond?

Fyi, the commonly advised koi stocking levels are 1000 gals per fish, although some would say 1000 gals for first fish, and 500 for ea. thereafter. If much higher than that I would guess that water quality issues may be a root cause to whatever else is going on . Too much is not known to really even guess - are other fish showing any signs of stress/illness (lethargy, clamped fins, gasping at the surface etc). Were new fish introduced recently who could be carrying disease?

If water is OK then its good to have a hospital tank with aeration/filtration where you can isolate and diagnose the sick fish and possibly medicate. Rubbermaid stock tanks work good for this purpose and its good to have one on hand anyway in case of other kinds of emergency.

A fish thats up around waterfall is possibly (probably?) there because thats where there would be more oxygen - so it may be gill flukes or something else affecting her ability to breathe AND/OR low oxygen levels overall. Thats where youd want to have her in a tank where you can control the conditions better and also apply treatments (if gill flukes then praziquantel).