well your pix are night time pix..we cant see behind your falls set up... the amount of koi you have in that small of a pond is going to need some good filtration. so your gonna need to get a 30 gallon or more tank, barrel, trash can(plastic outdoor) and build a biofilter. if you build a skippy style filter(upflow) your water enters at the top and leaves near the top. so as long as the output is higher than your falls you can locate it behind or away from the falls. check the DIY section and EQUIPMENT section for plans for filters. if youve got room dig out your upper pool area and locate your filter there.