Proper pipe size – Installing the proper size pipe in a koi pond is extremely important. To achieve good water quality you need to send all the water in the pond through a good filter at least once every 3 hours. Preferably, once every 2 hours. The point is, that if you build a large pond to support the large koi you plan to grow you are going to have to move a lot of water through the filter each minute. This will be impossible unless the pipe size is large enough. Study this chart and you will begin to realize what a difference pipe size makes.
See sacramentokoi.com design plans for more on proper pipe size.