Hi,
There are no short cuts or easy solutions.
1) you need to establish the cause.
2) once you have established the cause you can rectify the situation.
3) Be careful of the shot gun approach.
In order to establish the cause -
Create a time line - start with the number of deaths. When did this happen? Did all the fish die on the same day? If not the how many on which days?
In your time line - record what happened in your pond each day BEFORE the deaths. example: has your filtration system been working correctly? What was added to the pond water a. in the few days before the deaths b. in the weeks before the deaths?
Have you added and new koi to your collection? If so when. (and keep in mind the source).
In your time line - try to recall any changes in behaviour of your fish just before the deaths. examples: hanging around the waterfall, sitting on the bottom (depending on the temperature). Piping or gulping at the surface. Swimming with fins clamped etc
Check you maintenance schedule - do you clean the filter regularly, do you keep the floor clean. Are there any areas in your filters or pond that have a bad smell? or had a bad smell when you cleaned your system.
There are many possibilities for your fish deaths and more information is needed to arrive at a correct diagnosis. But this is a start.
Regards,
Chris