We did one of these for a SDKoi member as a club project, she was an older lady that had a few Koi only. What we did was set up the bottom drain to go to a 55 gal drum sank in the ground, and also did the same for the skimmer as well. This would give you settleing tanks that you can then use a 1/8th hp(
3960 GPH / 3.47 ft. Head@ 215 Watts) Lim's pump. Don't cap off the ventuires they can be used as Tpr's, and the bottom drain can be upgraded to a air dome as well. This would give much better circulation and keep debris on the move,and increase the DO and ORP.
Now from the pump you could add a Uv 25-40 watt in line to system(returned via Tpr's). For the filter we used a round tank app 150 gals and about 4' tall, we added Matela for the media. The water entered in from back and flowed through media to be gravity feed back to pond. Also by passed filter to send some flow to Tpr's. We made a small waterfall about 2' high stepped by stacking flagstone in layers. The filter was hidden by a lattace panel in front of system.
IMHO a very simple but funtioning system for 3-5 Koi about 18". I will see if this pic comes in or not as an example...from Lim's website. She is still useing this setup and its been over 5 yrs now.
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Koiman™ CDKPA 