TheFishGuy
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sounds like a great idea! I have done some more caculating, and turns out based on volume, I cant afford to loose anything in that area, and still have enough space for the koi, so my current idea, was where I needed a fence to keep the cats out, I would insteead build an above ground bog that spills into the small pond after the streamPerhaps you can build the new pond next to the old one and convert the old one into a bog filter.
Pull the water from the new pond into slotted pvc manifold pipes buried under pea gravel in the old pond (now a bog). The water flows up through the gravel and returns to the pond by way of a low point in the common wall between the pond and the bog. Grow plants in the gravel which will help remove excess nutrients. Your pond water will be clean and clear.