When choosing a pump for your pond project, you have the choice of either an external pond pump, or a submersible pump. As the names imply, an external pump sits outside of the pond in a dry location, whereas a submersible pump is submersed beneath the surface of the water. Whatever system you end up deciding on, be sure to purchase a reliable, good-quality pump that is able to run 24-7, 365 days per year. Since pumps often drive the ecosystem in a pond, if the main pump fails, water imbalances can quickly occur, endangering fish and other life in your pond.
Pond pumps run on a fairly simple principle. An electric motor is used to turn an impeller, which draws water in one side of the pump, and expels water out another. The amount of flow is determined by the size and power of the impeller, which in turn will require a certain power output from the electric motor.
External pond pumps are generally used to draw water through a skimmer, which helps filter out debris floating on the surface of the pond, before pumping the water onto further filtration stages. Submersible pumps could possibly be used in this application, but due to space limitations inside the skimmer, it is much easier to fit the water intake of an external pump inside the skimmer, rather than trying to fit an entire submersible pump directly inside the skimmer. That being said, if your pond is small, it may be possible to fit a submersible pump inside the skimmer depending on the size of the skimmer and pump you purchase.
Because external pond pumps are located outside of the pond, if the pump is elevated above the water level, initially there will be air in the pipe from the pond up to the pump. If you were to fill the pipe with water to remidy the situation, the water will naturally drain back into the pond because of gravity. To get around this, you must fit a non-return valve at the intake to prevent the water from draining out when the pump is turned off. After installation, or whenever the pipes are drained, it will be necessary to manually fill the piping with water so that the pump will be charged for use. Naturally this requirement can be ignored if your pump is below the level of the surface of the pond, in which case gravity will naturally charge the pump. In the case of a submersible pump, a non-return valve is not necessary due to the pump being submersed in the pond.
The amount of water in your pond will determine the size of pump you require. As a bare minimum for proper biological filtration, your pump should be capable of turning over the entire contents of your pond every two hours. If the flow rate is allowed to fall below this benchmark, either because of an insufficient pump, or because of a clogged pump, various problems can be introduced. These problems can include low oxygen levels, accelerated algae growth and higher than allowable ammonia and nitrite levels. Any of these problems are capable of endangering the fish and/or other life in your pond, so try your best to avoid them by starting with the right pump for your application. Also keep in mind that pump flow rates are measured assuming no strain on the pump due to pipe fittings, or gravity. Since any pond will include pipe fittings and a potential change in elevation from the pump intake to the pump system discharge (such as in the case of a waterfall), head pressure will be introduced to your pump system which will reduce the flow of your pump. To learn more about head pressure, please see our guide on pump head pressure.
Please keep in mind that pumping the entire contents of your pond every two hours is a minimum. If you plan on installing a waterfall or stream of some kind, you may need to pump more than this to achieve the desired effect. For this reason, when choosing an external pond pump, we always recommend going a little larger than required to plan for any tweaks or unexpected changes later.