This pond reminds me of my father-in-law's pond. His pond is about 1800 gallons and he has about 25 koi ranging from 8" to 26". It is a very beautifully-lanscaped pond with bonsai everywhere. Depth ranges from about 18" (yes, 18"!!) deep to about 3 feet deep.
Believe it or not, filtration consists of water flowing from the top of the pond (no bottom drain) down a waterfall and into a 150 gallon livestock watering tank that consists of just J-mats (no settling chamber or mechanical filtration; I have been his son-in-law for 3 years now and never once have I seen him clean the filter). The only aeration is the water falling about a foot into the filter tank. The water is then pumped from the bottom of the filter tank via a 1-hp Sequence pump and returned to the pond via a waterfall (turnover has to be at least 2 to 3 times per hour!). He also uses a 40-watt Aqua UV. He does not own any medications, no ORP or salinity or pH or GH or KH or ammonia/nitrite/nitrate meters/kits.
The pond is sloped and the deepest part has a 2" drain with 2" flexible pvc pipe that leads to his garden. Twice a week he opens a valve at the end of the PVC pipe, collects about 20 gal of the dirty water, and uses it to water his bonsai. The autorefill then replaces the lost water. This means he only changes 40 gallons' worth of water every week and yet his water is crystal clear and his koi feed like there's no tomorrow. The sad part to all this is I have 14 koi ranging from 10" to 24" and yet his koi seem to grow faster than mine.
Lessons I learned: (1) good husbandry is one of the commandments of koi keeping; (2) you can't beat old men with experience; (3) there's definitely more than one way to keep koi.