Make your Bio Filter the base for your waterfall. The challenge then is to make it look natural. That takes time due to the necessary growth of vines, bamboo and any other natural greenery to hide parts of the construction.
I built mine 6 months ago (108 degrees when we did it) out of a 55 gallon plastic drum. I'm supplying it with a 1150 GPH Beckett pump. The filter media is 7000 cut pieces of black irrigation tubing, half inch in diameter cut 1 1/2" long. I put a 2" dump valve in the bottom, it takes only a minute to clean the filter by just opening the dump valve.
My pond looked like pea soup before turning this on. After 2-3 days, the water was crystal clear. For the first time ever, I could see sand grains on the bottom.
This looks terrible until you hide it by building more structure around it to make it look more natural. Mine is getting there with the growth of the vines and bamboo around it.
I have many more photos that I will add to my web site some day. I'm thinking I need to set up yet another web site just for this Forum and the Koi.