Submersible pond pumps are totally in the pond, meant for small ponds and are very economical to use. The pump is sized by GPH or gallons per hour, how much volume or flow the pump is capable of producing.
A pond holding 300 gallons will require a minimum 300 GPH pump, giving you a turnover rate of once per hour. If you add a waterfall, you will need a larger pump. Taking into consideration other items such as filters, and UV clarifiers will also increase the GPH required.
You may find that installing a second pump for your waterfall will lessen the stress on your primary pump. Length of piping, number of fittings and bends all stress the pump, shortening its life expectancy.
When looking for a submersible pump, purchase one that carries a two year warranty and comes equipped with longer cables to minimize the need to join cables. The cables should be constructed with 3 conductor wire, so that there is no doubt that you have a pond pump and not an aquarium pump, which typically uses 2 conductor wire. As always, when working with electric, consult your country's code requirements.
The newer magnetic drive pumps, also known as a floating rotor pumps are more environmentally friendly than some of the older models, because they use the water to lubricate the pump, therefore no chance for oil to contaminate the pond. There is no shaft seal, the coil is epoxied so that it is not prone to water seepage, and the pump uses much less energy than the older magnetic drive pumps. Place your submersible pond pump into your pond so that it will not pull debris from the bottom into the pump housing.
If your pond features a waterfall, plan on up sizing your pump 20-40% to accommodate resistance of piping and fittings. The larger pump will still provide a good flow even when your pre-filter is clogging.
Pre-filter equipped waterfall pumps will not clog as long as they are large enough but the small pumps require daily cleaning and are not recommended for your waterfall. The pump is easy to install but not very energy efficient. Therefore, if you want a waterfall pump, consider installing an external pump.