A proper pond aerator is vital to having a healthy koi pond, water garden or lake no matter what time of year it is.  Use a pond air pump during hot weather to provide the necessary oxygen.  During the winter an air pump will keep an opening in the ice, add oxygen and help gas off toxins.

Any system will benefit from adding an aerator.  An air pump will also help the growth rate of your fish, both koi and goldfish.  Adding an aerator will also help the biological process within your pond.  Biological filtration requires  large amounts of oxygen so pond air pumps should be used.

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. 

The amount of oxygen water can hold is dependent upon atmospheric pressure, salinity and temperature. Water can hold less oxygen as altitude increases. Salinity is not important for most freshwater fish producers. The most important factor is water temperature. As temperature increases, water can hold less oxygen. Most low oxygen problems occur from June through September. The reasons for this are: 

• Water can hold less oxygen as it becomes warmer.
• Respiration rates of both plants and fish increase with the warmer water, so more oxygen is used.
• Summer's still, hazy or cloudy days may reduce the amount of oxygen produced.
• Large amounts of feed given to fish at this time of year result in large quantities of fish waste which create a higher demand for oxygen.