This article was first written for and published by Koi Magazine in 2009
There is a school of thought that says that you can never have too much aeration in a Koi pond but it is not one that I adhere to as once the water is 100% saturated with oxygen there is no point adding more – any further addition of air is a waste of energy and money. However, aeration is important as it is not only the fish that need to breathe in your pond but also any plants in it and, if you have green water, the phytoplankton that make up the green water will too. Aeration is also important for the process of filtration as the good bacteria in your filter need oxygen to survive and nitrogen and free ammonia need to be released into the air.
Aeration doesn’t just involve the transfer of oxygen from the air to a living organisms but also the movement of carbon dioxide and nitrogen gas from living organisms to the water, then to the air, and vice versa (organisms being fish, bacteria, plants, algae). The concentration of gases in the water is influenced by many factors such as the water temperature and other dissolved compounds in the water and also suspended solids. As the water temperature rises this increases the resistance to diffusion, in short, the warmer the water, the less oxygen can be held in the water which is why it is particularly important to provide good aeration in the summer.
The limiting factor for the rate of transfer of gases into the water, and therefore those available to the fish, is the surface to air contact. The greater the surface area of the water, the greater the gas exchange. Bubbling air is a good way of introducing oxygen into the water and spraying water or crashing the water (for example via a trickle tower or a waterfall) back into the pond allows the dissolved carbon dioxide and nitrogen to gas off giving more space for oxygen to be dissolved, so the more bubbles, spray or crashing water the more gas exchange and the more oxygen in the water.
By providing adequate aeration you can be sure that the water holds as much oxygen as possible but there are other factors that must be taken into account. The turbulence caused by bubbling too much air into the water can prevent the fish from relaxing by removing calm areas of water where they can rest therefore causing stress. It can also prevent suspended solids from settling out and passing through the bottom drain – smaller bubbles however provide better aeration and less turbulence. As the air bubble diffuses into the water it gets smaller and it should disappear before it reaches the surface in order to be 100% effective – any bubble that breaks the surface is a waste of energy. In these energy conscious times it must be taken into consideration that pumping air or water takes energy. Trickle towers, whilst being the most efficient way to provide aeration and excellent at degassing, are also the most energy hungry. My own feeling is that bubbling air through air stones or a diffuser and spraying or crashing the returning water back into the pond is more than enough to provide a healthy environment for your Koi and as is also the cheapest and most energy efficient way of aerating a pond.