I am concerned that you have too many fish for your size pond. Koi grow quickly and if your pond is undersized, you will hit a tipping point... from "everything is great!" to "my fish are all dying!" literally overnight. The first thing I would suggest is to re-home some fish.... or all of them. With a pond that size you could have dozens of goldfish, or a small handful of big koi. Naturally the choice is yours, but if you don't choose, I fear nature will choose for you. Now on to your real question...
We've had a bog filter on our pond since day one, and it's done great to keep the water healthy and the fish happy. However, we did start to notice an increase in the amount of tiny floating debris in the pond. (We don't have a skimmer and didn't have a biofalls) so essentially we had great biological filtration but inadequate mechanical filtration. We could catch the big debris, but the tiny stuff just kept going round and round.
Two years ago we bought a simple box filter with four filter pads and a pump - didn't look pretty sitting next to the pond, but made a HUGE difference in the amount of floating debris. We needed a rebuild on our waterfall, so we talked to our pond builder about the issue we had with floating debris and he suggested adding a biofalls - not for the biological filtration, but to have a place to add filter pads to catch the tiny stuff. About 2/3 of the water now pumps through the bog and 1/3 runs through the biofalls. Perfect solution for our situation.
So long story short, while I'm a big bog fan, in our case I think we don't have enough settlement time in our bog for the fine debris to settle to the bottom. Adding a biofalls solved that issue for us.
