How many gallons does a fish tank have to be to keep a koi fish in?

From numerous websites, I've seen answers that range from 50 gallons, 100 gallons and as much as 1000 liters (260+ gallons)! I use the 100 gallons per koi estimate and have had no problems for the past 4 years of having my pond. I have ten koi in a 1000 gallon pond and a second 1500 gallon pond that is connected with a stream that has 14 koi. Two waterfall boxes, one skimmer and two 3600 gallon pumps running it. PH is always around 7.2 and haven't had any diseases, etc occur in the 4 years.