As a former landscaper who built many ornamental ponds I would advise against the Koi unless you get black ones. They are simply too visible for predators. Odds are they will get eaten. Not a 100% sure but....

I am about to build my own pond. I am looking at about 50-60 ft across and about 100 ft long average depth 8-10 ft. in a kidney bean shape. Not sure what the acreage is. I plan on spending $1000-$1500 on equipment but I will be digging it myself.
Ck. the web and get some free estimates.
Good Luck!!

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