Goldfish pond water pumps need to pump the water volume of a pond through the filter every two hours.
Pump volume or water output figures are based on the bare pump sitting in clean water with nothing connected except the power lead.
This isn’t how the pump is going to be used. Factors that will affect those output figures are:
If you are planning a fountain or waterfall, you will need a considerably larger water pump to offset the water volume drop as the height above the pond increases. Most water pumps detail the water volume loss based on head height above the pond water level.
If the waterfall or fountain is going to take a lot of water consider running a dedicated water pump.
The bottom line is the same as for filters…I double the pump water volume I need to allow for all the factors that I know will decrease the actual water volume the pump will deliver.
Combined filter/pump units can be treated the same way by buying a unit twice the size needed for your pond.
You can always slow a pump that is too big down, but you can’t increase the water volume on a pump that is too small.
For a 100 gallon pond, buy a 200 gallon rated pond filter, and buy a water pump that pumps 100 gallons per hour.
The majority of pond water pumps are submersible and sit on the bottom of your pond.
Most need to stay submerged because the water is used for cooling. Place the pump on a platform several inches off the bottom.
These few inches of water are a safety factor if the system develops a leak and the pump empties all the water out of the pond. It is easier to replace a burnt out water pump than expensive goldfish.
Ideally, you should buy your water pump and filter from the same manufacturer because the fittings will be guaranteed to match. The only thing you need is some flexible non-kink non-toxic PVC tubing to connect everything together.
Tubing supplied by filter and pump manufacturers is usually UV protected to stop it from becoming brittle over time from sunlight exposure.
If you want to buy your water pump from Amazon.com or see what is available, click on the link Pond Water Pumps.
If you need some PVC tubing you might as well get it at the same time. Match it to the size of your pump outlet. Don't go smaller in size as it increases the pressure which decreases water flow. Click on the link Water Pump Tubing.
You will need some hose clamps as well. Buy only stainless steel clamps.
Don't over-tighten them on the plastic fittings, just nip them up lightly and then pressure test your system once it is all assembled.