For many people, it's not really a question of whether to use a submersible pump vs. an external pump, primarily because most people have smaller ponds and are used to using a submersible pump. Submersible Pond Pumps are easy to install, and they are pretty reliable.
The Benefits of Submersible Pond Pumps.
For understandable reasons, a submersible pump derives its name because it is designed to be placed in the pond, and submerged under the water.
Submersible Pond Pumps are the easiest of all pumps to install. Installations require no more than attaching a hose to the waterfall, or to a submersible filter, and then just drop the submersible pump in the water and plug it in - and you’re ready to do.
Submersible Pond Pumps range in size, or gallons per hour, from 50 GPH all the way up to 50,000 gallons per hour, but for most ponds - pumps anywhere from 350 GPH to 4000 GPH will be sufficient.
Submersible pumps are best suited to small to medium sized ponds with roughly 200 - 800 gallons of water. These are often the best choice because they connect easily to most of the submersible filters that are available for small to medium sized ponds. Flow rates range from 250 GPH to as high as 4,100 GPH for a larger waterfall pump. Generally, the ideal pump will circulate the total volume of the pond at least once per hour. For example, if you pond is 500 gallons, you would normally want to go with at least a 500 GPH pump.