Suppose there are 10 adult koi that are 20" (about 4-5 pounds each), living in a pond that is well aeriated by fountains and/or waterfalls, but the water is unfiltered. How big does this hypothetical pond need to be? 10,000 gallons? 100,000 gallons? 1000 square foot surface area? 10,000 square foot surface area?

This pond and these koi do not exist. This question is for educational purposes only. Thanks.