Dissolved oxygen is vital in the pond. It is used by fish, plants, pond wildlife and microorganisms, including filter bacteria, for respiration.

Gases, including oxygen and carbon dioxide, defuse in and out of the pond through the surface where the air and the water meet. The greater the surface area of the pond, the more rapidly gases can exchange. The surface area of a pond can be increased by adding a fountain or a waterfall thus rippling the surface of the water and increasing its area. Gases will also diffuse into water by contact with a submerged bubble’s surface area, so adding an air pump or a venturi to the pond can be extremely beneficial. At normal pressure there is a limit to the amount of oxygen a body of water will hold, this is proportionate to the amount of oxygen in the air around the pond and is governed by the temperature of the water. Warm water can not hold as much oxygen as cooler water.

As pond temperatures rise in the summer, the amount of dissolved oxygen the water can hold is reduced. This is not ideal in the pond as fish have a higher metabolism and require more oxygen as temperatures rise. Because the fish have a higher metabolism they are producing more waste, so there are more bacteria required to remove the waste, and these bacteria also need oxygen. At night plants also absorb oxygen when they are not photosynthesising. It is easy to see how a pond on a warm summer night can soon become starved of oxygen and the fish can suffocate. It is therefore extremely important to use air pumps, or to keep fountains and waterfalls running, especially through the night, to increase the pond’s surface area and allow the quickly depleting oxygen to be replaced.