I'll echo Mark's comments, but I wonder about the Koi output! Anyway, filtration is often undersized, I use biological filters, but cannot have a lot of fish without making the filters huge. If I wanted a lot, I'd either have a big pond or outside filters like a swimming pool. The biological filter worked fine for me, but with only a few fish. I was keeping plants too, so I think that was my only choice.
The manufacturers of these filters size them to the pond in gallons.
Regards, Greg