I use lava rocks for my bio filter. I am not sure how your filter is setup but give it a try, they are cheap and have a good surface area.
Also, I only have a 100 gallon pond with 2 10" Koi and 3 4". I am fully aware that they can't live in there forever, but I plan on moving next year and building a bigger pond. Keep in mind that Koi have been know to live 25 to 35 year, and get up to 3 FEET long. Believe me they can become huge, but I am sure you knew that before you bought them.