Your koi pond should be optimized to maintenance free as humanly possible.
Hence the experts recommend laying out your pond bottom drains correctly. Keeping in mind that the drains will only draw from a radius of 4-6 feet.
Moreover, the slope should be within at a minimum of 1 inch per foot, but a 2-3 inches is even better.
The expert koi keepers recommend installing a skimmer at the furthest point from your waterfall, if possible.
Jets are strongly recommended in order to help direct the debris to the skimmer. They also help prevent dead areas in your pond. Further, the jets help exercise your koi as the swim against the current.