Having a pond full of beautiful Koi fish is a source of joy and an achievement to be admired. You see, Koi need some very special needs in order to remain healthy and grow. Besides food, Koi require highly oxygenated water. It is simply not a case of digging a hole, lining it and putting in the fish. Koi need lots of oxygen. Because they use the oxygen in the water rather rapidly it needs to be replenished all the time. I have some suggestions for how to have a Koi fish pond with lots of oxygenated water.
There are several ways to oxygenate the water in a Koi fish pond. One way is to have enough surface area of the pond to allow a natural exchange of oxygen happen between the air and the water. Most pond systems have a pump and filter. The larger the pond or number of fish in the pond is what dictates the size of the pump. Most Japanese gardens with large Koi ponds use industrial pumps and have a pump house. Smaller backyard ponds use surface area and pump that operates a fountain or waterfall. The motion of the water speeds up the exchange of the oxygen from the air into the water. The faster the motion, the more the exchange.
Another thing that all Koi ponds must have is an area deeper than 3 feet deep. This area is what the fish will need to escape from predatory birds that like the taste of fresh fish.
You can now see that the shape and depth of the pond contributes greatly to the success or failure of the Koi fish. The ideal Koi fish pond will have a large surface area, a deep spot for the fish to escape to from predators and heat during a hot summer day and have ample oxygen in the water.