Aeration is a vital part of a koi pond. Not only does it help with oxygen, but it also aids in circulating the water. Aeration is also very important for the biological filter with separate air stones to assist the beneficial bacteria to flourish therefore giving you healthier water parameters.
Oxygen in ponds comes from two sources--photosynthesis and diffusion from the air. The most important source, photosynthesis, is the process plants use for manufacturing food.
In the presence of sunlight, plants (especially algae) add oxygen to water as a by-product of photosynthesis. At night, no oxygen is produced, but respiration of algae, fish and bacteria continues to remove oxygen from the water.
Most of the time there is a desirable balance between how much oxygen is produced and how much is used, but under some conditions, the balance can be upset, and the oxygen concentration becomes low enough to stress or kill fish.
The amount of oxygen in pond water can vary considerably from pond to pond and from hour to hour. Typically, however, oxygen concentrations are lowest at dawn and highest during late afternoon.