Aeration is a very crucial element in a koi pond. Every living thing in the pond requires a constant supply of oxygen. The koi, plants and even the bacteria in the filter need ample oxygen.
The best way to aerate a koi pond or water garden is to constantly draw water from the bottom of the pond and return it to the top. This takes the poorly aerated water at the bottom and sends it up to come in contact with the oxygen at the surface. Constant circulation, 24 hours a day, is needed to maintain proper aeration in a koi pond.
A common method of adding oxygen to a water garden is a waterfall. Aeration takes place though oxygen exchange. Anytime water and air come into contact, oxygen exchange takes place. The greatest amount of contact the water in the pond will have is on the surface. The reason a waterfall will add oxygen is because it breaks the surface and helps circulate the water on the top. The deeper the water from the falls penetrate the surface, the better the aeration.
Another easy way to aerate a koi pond is to use an air pump and air stones or diffuser. The air diffusers are placed at the bottom of the pond and they create a rising column of air bubbles. Like a waterfall, the bubbles themselves don't add much aeration, but what they do is create a water column bringing water from the bottom, to the top where it can get aerated at the surface. So you improve aeration and circulation at the same time.