Hmmm, do you have a local koi place that specializes in koi only? If you do, phone them and tell them what is wrong. They may be able to help you.
Bleeding of gills, Fins rotting, White mucus all over fish? Hmmm. You could have a combination of multiple koi ailments.
First thing to do.
Test for all water parameters!
Nitrate?
Nitrite?
Ammonia?
TDS? If you have it.
----
What is your stocking rate?