A pond pump circulates, filters, and oxygenates the stream in your pond. It is the heart in the pond which is responsible for keeping everything going. There are 3 main parts with the pump.

They are the impellor that spins and drives the river, water intake pipe along with the outlet pipe. The intake pipe is attached with filters which clean the stream before it enters the pump. It is the outlet pipe which moves the stream above the surface, with the waterfall and back into it. The water with your pond needs to be circulated once every hour.
Circulating and Oxygenating
In addition to providing oxygen to the fish the pump's most essential job should be to provide oxygen to bacteria within the fish pond bio filter. It is the pumps job to continuously replenish it with freshly oxygenated water. The bacteria need oxygen from circulating water so as to live and do their job of purifying pond water. These bacteria waste large amounts of dissolved oxygen which includes to be replaced.
Facts about water and oxygen
• Pumps keep water constantly circulating throughout the system.
• Fountains are certainly not wasteful of water. The only water lost is via evaporation
• Water absorbs oxygen at any interface between air and water
• Cold water has more oxygen than warm water
• Water has more oxygen in daytime than at night
Filtering
A water pond pump also filters algae and debris.

It pulls the stream through its intake after which it is pulled by using a filter device. After the stream is filtered it may be returned straight to water-feature or by having a waterfall.
Water Levels
Pumps are almost maintenance free. If you maintain filter clean the pump continue running properly. One important thing to remember should be to ensure the stream level inside the pond stays over the submersible pump. During the hot summer months the lake will evaporate which enables it to be a problem, specially in smaller ponds. Storms may also damage your waterfall and cause water to own out on the pond. This will also cause your water level dropping. If water gets too low you are going to hear a loud chugging or growling noise originating from your pump. This can damage your pump if you can't catch it quick enough and it'll result in replacing your pump.
Garden pond pumps work harder when they have to go water up and via a waterfall. This greatly supports the oxygenation process. Be sure to account for your extra work your pump does to get a waterfall when selecting your pump.
Your pond water pump should run round the clock, every day in the year. If the pump doesn't run and also the bacteria die then this oxygenation process is stopped. This could be dangerous for your fish. It could cause much of your fish to die.