when doing a water change be sure to use a water conditioner to make tap water safe if you have chlorine in your water.

First, you need to start doing regular water changes. 10% to 20% weekly is usually wise. The fact that you have high ammonia and no nitrites means you do not have a cycled bio filter. Is this a brand new pond or do you just not have a filter?

when doing water changes, make sure you use dechlor unless you are on well water. Second you need to add an ammonia binder to prevent the ammonia from harming the koi. There are several good brands...ChloramX dry is my favorite since it is the cheapest per gallon treated. It works as both dechlor and ammonia binder.

but water changes and ammonia binders are just a band-aid. we need to look at your filtration to see what is wrong or this problem will never go away. How big is your pond? How many fish? What size pump? What type of filter? Do you have a bottom drain? Skimmer? Rock bottom?