I have been keeping koi for around 20 years, my first pond was a 1200 gallon in ground liner pond, pump fed to a kockney koi multi bay filter.
Then I built my current pond in 2016 / 2017 which is 3700 gallons gravity fed system. The filter is a block built fluid bed with settlement, jap mat for mechanical and fluid K1 for bio. Returning via 2 super fish 12000 pumps to 2 mid water returns.
The skimmer is taken through another super fish pump, through a 55 watt EA uv clarifier.
I have 16 fish ranging from 26cm to 65cm. Including Showa, Sanke, Matsuba, Shiro, Chagoi and more.