Here is a full overview and walk through of my DIY 4,000 liner koi pond, including details of bottom drains, construction, RDF pre-filter, shower filter, moving bed filter, aeration, and automatic water changer. Please feel free to reach out with any questions, thanks!
Full build thread can be seen here: https://www.koiphen.com/forums/showthread.php?164973-Getting-back-into-koi-4000g-DIY-build-in-a-tiny-yard