Aeration is key to a healthy Koi Pond, all fish need proper aeration to survive. Extra Aeration  is needed during the warmer months for at least two reasons: 1) fish are more active in warmer water so they use a greater amount of oxygen and 2) warm water doesn't hold oxygen as well as cold water.

Aeration can be produced by air breaking through the waters surface. One way of doing that would be using an air pump, such as the Fusion. To properly work the Fusion you need to get some supplies that are sold separately such as air hose and air stones. To determine how much tubing and stones you will need you have to take some things into consideration 1) how big is the body of water in which you are trying to aerate, 2)how many stones are you going to use, and 3) where are you going to place you stones . Please keep in mind that you will want to separate your Air stones so you can effectively aerate your pond and that you will need enough tubing to go from the pump to between 1-3 feet below the water surface (the deeper you go the greater area the stone will aerate). 

Another way to provide aeration to you pond is by having and running your waterfall. Some people decide not to run there waterfall at night, usually this is fine. Although in the warmer months it is recommended that you either run it as much as possible or provide another source of oxygen for your fish.