Information - Choosing the right size Filter & Pump

Accounting for environmental factors
Now that you have calculated the volume, one must realise that there are environmental factors that affect the ability of your filter and pump to produce clean, clear water. These must be calculated into the pond volume and is otherwise known as the effective pond volume. The following are certain factors used in the calculation

Average pond water depth is less than 75cm: + 25 %
Ponds located in full sunshine: + 25 %
Ponds located in subtropical climate (e.g. Queensland, N.T.): + 35 %
Ponds located in temperate climate (e.g. NSW, Victoria. SA.): + 15 %
Ponds located in Southern climate (e.g. Tasmania): + 0 %

Determining the flow you want
If you have a pond that is under a few thousand gallons you may want to turn it over 2 to 3 times per hours. Since smaller pumps will use a little amount of electricity it is still affordable. If it is a larger pond you may want to turn it over only once every 2 hours to three hours.

Many Japanese breeders will tell you that the real volume of water a fish lives in is determined by multiplying the pump's flow per hour times 24 hours per day. For instance, 10,000l/hour yields a "real volume" of 240,000 litres that the koi actually live in, regardless of the actual size of the pond. This says that the water pump's output is more important than the size of the pond. Thus it is wise to turn over a small pond as much as you can afford. The flow rate is a measure that is expressed as volume (litre) over time (hr) ie 3000l/hr. The flow rate that you wish to have will determine your pipe size and pump size.