External pond pumps are best suited to medium to larger sized ponds with 1000 gallons of water or more. These are often the best choice because they are typically more energy efficient and can handle larger water flows.
External Pumps are also designed to work well with most biological filters, which can cause significant backpressure on the pump. These pumps usually come with a priming pot and leaf strainer which is easily accessable without having to get your hands wet.
Flow rates range from 1800 GPH to as high as 7000 GPH for a larger pump. Generally, the ideal external pump will circulate the total volume of the pond at least once per hour. For example, if you pond is 2000 gallons, you would want to go with at least a 2000 GPH pump.