Pond Pumps
There is a wide variety of pond pumps to choose from. First you will need to choose between a submersible pump and an external pond pump. Most pond owners will purchase a submersible pond pump because of the ease of use and the lower cost of the pump.
There are a few things to consider when choosing your type of pump. Whether it is a mag drive, direct drive or hy-drive. You will want to consider the cost of the pump and specially the warranty. Direct drives will give you more flow for a longer or higher distance. A mag drive will generally save money on your electric bill and a hy-drive will be in between. 
Placement of your Water Garden Pump 8 to 12 inches off the bottom of the pond makes maintenance easier to get to your pump and there is less clogging. Also if ever there is a leak on the filter, all your  water will not be pumped onto the garden, thus leaving some water in the pool for the fish to survive.
When having a pond pump make sure the flow can be handled by the pond filter. If the flow is too heavy you can make a bypass in front of your filtration system or use a valve to turn down the flow right after the pump. This will insure that your filter is capable of handling the flow for its size and if the filter has a UV you will want to keep the water flow within the ultraviolet clarifiers most efficient flow rate.
Choosing the correct pond tubing can mean more or less flow. The bigger diameter of the hose the less friction you have and the pumps will be able to work easier and provide a greater flow then if the tubing was a smaller diameter.
Up to around a 4000 gallon pond you will want to turn the water over at least once an hour for best results, more often when it is a smaller pond.
When using  waterfall pond pumps for a filter falls or waterfall feature you will want 400 Litres pr hour for every inch of width of weir as that will give you a nice waterfall. If you want a lot of water then you would want up to 900 Litres per hour for every inch of the weir.
You will find many manufactures of pond pumps including Oasis Water Gardens, Fish Mate, Laguna, Blagdon, Oase, Lotus, Otter, Sea Lion, Project.