For a Waterfall: Decide the width of your spillway and determine the vertical distance from the top of the waterfall to the pond surface. Every inch of width of the spillway requires about 100 gallons per hour of water flow at the height of the waterfall. For example: if you want an eight-inch wide stream over a waterfall that is four feet above the pond surface, you need a pump with a flow rate of at least 800 GPH at a height of four feet. All pump packaging is labeled for water volume specifications at specific pumping heights.

For Filtration: Calculate the water volume in your pond. The lowest powered pump you may use is approximately half that volume. A 750 gallon pond needs a minimum of 375 gallons of water moving through the filter every hour. The pond will turn completely every two hours. Read the filter specifications to figure out the maximum volume of water that can pass through the filter. Typically, turning the volume of your pond hourly to every hour and one half is helpful.
 
If you need to move water in a pond, whether it is for a fountain, a waterfall or a filter you will need a pump. All these functions require different pump characteristics it is necessary to have pumps with different characteristics.

Fountain Pumps need to be stable and be able to operate efficiently for long periods without the need for regular maintenance. However they have to pump semi clean water to avoid blocking the fountain jets. Waterfall pumps need to be able to pump large volumes of water efficiently. This water however does not need to be free of solids. Finally filter pumps need to be able to operate for long periods of time without blocking, they also need to be able to pass solids to the filter to enable them to be removed from the pond. As they are part of the life support system for the pond they need to be very reliable.