These are selection criteria to choosing a pump:

• Calculate your pond volume in litres:- Estimate the surface area of the pond in square metres(square feet) then multiply this by the average depth, not the maximum depth, of the pond in metres(feet). This will give the pond volume in cubic metres(cubic feet) which is then multiplied by 1000(28.4) to give the approximate pond volume in Litres.
• If you are going to install a filter make sure it is designed for a pond of your size and be guided by the recommended flow rate. Be aware that if the filter is to be positioned above water level check that the pumps performance at the total height above the water level (Head)that the water needs to be raised It is worth adding approx 10cm to this height for every 1m of pipe run to allow for the resistance of the hose. As a general rule try to filter the entire volume of the pond every 1-2 hours.
• Often a Ultraviolet steriliser is used in conjunction with a filter system. Make sure it is compatible with your filter system and your fish stocking rates, not forgetting that fish will grow ! A UV system for a 10000 litre pond which is heavily planted, in partial shade and with only a few goldfish will not be able to cope with the same size pond devoid of plants, in full sun with several large Koi carp.
• When a waterfall is to be installed it will require about 265 litres per hour for every 25mm(1inch) width of the lip of the return to give a thin continuous sheet of water. Again the head and length of hose should be allowed for.
• Fountain pumps are a straightforward choice and is purely down to your preference of spray pattern and to make sure that all the water falls back onto the pond surface and not emptying your pond especially important in windy weather.
• As a general rule it is better to have a pump with a greater capacity than you need as it is a fairly simple job to restrict the flow to match your needs but you cannot extract more performance than the pump is designed for.