Iām hoping i had a genius level idea with biochar :).
It might help but purple non sulphur bacteria will clean it out beautifully and its good for the plants and fish too :
https://www.algaebarn.com/shop/aquarium-supplies/pns-probio/
Its also in EM, bokashi and worms. So you could inoculate the biochar with that first, when you put it in it should inoculate the pond. Just start smallish and build up if you are using the em and bokashi to ensure the Koi have adequate oxygen in the water.
Discussion
It's genius just needs inoculating š . It is another rabbit hole to explore for your biochar experience. Once you have the pond inoculated you can use the pond to inoculate your new biochar so you have gone full circle š.