If you keep Koi or other pond fish, you might want to consider adding a pond air pump to provide sufficient air to the pond and to the filtration units. Your Koi could be a lot of trouble if the oxygen level of the pond drops below what is required to sustain life.
An air pump is needed especially if you have a large population of fish and most especially if your power fails frequently. If you do purchase an air pump, get the largest that you can afford so that it will pump air into the depths. If you have one that only pumps to 3 feet, little is gained if your pond is 4 feet or deeper.
Location of the air pump is important, too. It needs a fresh air source. Be sure that the pump is drawing in quality fresh air, no fumes or odors. There are piston drive and diaphragm models available, depending on your preference and wallet.
During hot weather the air pump helps add oxygen to the pond and during cold weather, the pump will help keep an opening in the pond for oxygen to enter and gas and other toxins to escape.
Aerators, as these pumps are known, also help your fish to grow and will aid your bio filter get the need oxygen to do its job. As the temperature of the water increases, it holds less oxygen just when the fish need more oxygen. The pump insures that oxygen will reach to the bottom of the pond. Look for pond air pumps that are corrosion resistant and require cleaning only once or twice a year. Be sure to check seals to guard against oil leaks.
In the end, if you have a properly sized water fall pump or just a pond pump, you may not need an air pump to get oxygen to the bottom of your pond.