Air pumps for ponds create healthier environments for Koi and other fish. Fish use their gills to extract dissolved oxygen from the water. While each kind of fish has unique oxygen requirements and tolerances, most fish thrive in water with dissolved oxygen levels at five parts per million or higher. When the dissolved oxygen concentration falls below this level, fish become uncomfortable. Most species will die if the dissolved oxygen drops below 2.5 to two parts per million. As the oxygen level drops, fish may be observed gasping at the pond surface or gathering in areas of the pond where the higher levels of dissolved oxygen may be found, such as under waterfalls.
The amount of oxygen that will dissolve in water depends on the purity of the water, the available surface area, and the temperature. Gasses become less soluble as the temperature of the water increases. A pond at 70 degrees Fahrenheit can have a dissolved oxygen content of up to nine parts per million. This is the saturation point for oxygen at this temperature. There is literally no more room in the water for oxygen to dissolve. Any additional oxygen will remain a bubble of gas and be useless to fish.
However, ponds with fish rarely remain at the saturation level without help. The decomposition of biological waste uses oxygen that is taken from the available dissolved oxygen content. As fish waste accumulates in the pond, the dissolved oxygen level drops.
When not aided by air pumps for ponds, depleted oxygen levels are replenished wherever the water comes into contact with the air. Larger surface-to-air contact areas provide greater opportunity for oxygen to diffuse into the water. Agitation of the surface, such as water splashing back into the pond from a waterfall or a fountain, increases the available surface area markedly. Even ripples created as water moves over or around stones in a stream provide increased surface area.
Pond air pumps release millions of tiny air bubbles into the water. Oxygen can dissolved into the water all around the surface of each of these tiny bubbles. Underwater plants, which release small bubbles of oxygen as part of the photosynthesis process, also increase the dissolved oxygen content of the pond.
In addition to creating a healthier environment for fish, a pond air pump will also increase the effectiveness of the biological filter. Like fish, nitrifying bacteria require oxygen to thrive.