Best for medium to large ponds with over 1,000 gallons, external pond pumps are more energy efficient when compared to submersibles. These pumps work with biological filters better than submersible pumps because of the back pressure that the filter places on the pump. These are also easier to clean because the housing is outside of the pond, therefore you will not get your hands wet while cleaning and servicing external pumps. These pumps are also designed to circulate the water at least once an hour, so if your pond holds 1500 gallons, you will need at least a 1500 GPH pump.
In terms of energy efficiency, these pumps are the best and can pump larger volumes of water. Considering that the pump will need to run 24-7, it is a good idea to purchase the best rated pump that you can afford and to be sure that it can handle the load.
External pond pumps ideally will circulate your pond water at least once an hour. For example, say your pond is 2500 gallon capacity, you will need a pump with at least 2500 GPH capacity in order to "turn over" the pond this frequently. The external pump will be a longer-lasting, less expensive to operate than the submersible running the same amount of GPH. The pump itself costs more, but because it is so energy efficient, your electric bill will not suffer as much.
The external pump is also easy to camouflage because it sets low to the ground. It can be hidden by bushes as long as it is not placed too far from the pond or has to pull water uphill more than a few feet.
High volume garden pond pumps are generally used with large waterfalls in ponds that require 1200 to over 5000 GPH. These pumps are usually external and if be installed below the pond surface, the pump will be self priming. If you cannot install the pump in what is known as a "flooded suction", set the pump as low as possible and as near to the pond as is feasible. Shelter your high volume garden pond pump and motor from the elements to help it last longer, but do be sure that whatever shelter you use provides air circulation to keep the pump cool. Always be sure to check the water inside of the pump and lines because these pumps are meant to pump water and should not pump air. Installing shut off valves before the pump and after will allow you to easily remove the pump from the lines without the hassle of draining your system.