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What Size Filter?

Filter Size Calculator

Brief Introduction:

Although not a dishonest act, many filter system manufacturers convey the illusion that their pond filters will happily cope with say a 2,000 gallon pond regardless of fishload or environmental conditions. This alas is simply not true, and Cloverleaf have gone to the degree of carefully formulating a series of helpful guidelines which should greatly assist in the pondkeeper establishing a sustainable and healthy pond with a filter of the correct size.


The water/pond capacities quoted by Cloverleaf within the specification of each model are based upon a series of typical fixed parameters, and where these are different a new calculation should be made with the applicable facts at hand. The Cloverleaf parameters are based upon: 1/ Fish loads not exceeding 200mm (8 inches) for every 1,000 litres (222 gallons) of pond water. 2/ Pond being greater than 800mm (31 inches) deep. 3/ Fish are only fed once a day. 4/ The pond receives no more than 6 hours of sunshine each day.

Note: The following calculations/equations are based upon a pond with a content of 3,000 litres (666 gallons).

Pond Depth:

If the pond is shallower than 800mm (31 inches), then an allowance of 1% should be added for every 10mm reduction in depth. Assuming then that the notional example pond is only 500mm deep, a difference of 300mm applies - which divided by 10mm generates a sum of 30. This then represents the addition of 30% - obviously illustrating that shallow ponds are not ideal.

Fish Stocking:

If the total “fish length” (excluding tail dimensions) is greater than 200mm per 1,000 litre of pond water then a related percentage allowance must be added to the calculations. With the example 3,000 litre pond, this is capable of sustaining a fish load totalling 600mm in length, however for the exercise we shall assume that in reality the length totals 750mm - which equates to a 25% increase beyond the normal datum. (This in turn translates to the 3,000 litre pond being reviewed as 3,750 litres).

Fish Feeding:

If fish are fed more than once a day, then a 10% allowance should be added for each additional feed. So for our example pond calculation we will include for feeding the fish twice a day, and thus add on an extra 10%. (Equating to an addition of 300 litres to the notional 3,000 litre pond).

Direct Sunlight:

If the pond receives more than 6 hours of direct sunlight a day, then a 5% allowance needs to be added for every further hour. For this example we will assume the pond receives 7 hours of sun each day, and as a consequence 5% must be added. (Meaning the example 3,000 litre pond should be reviewed as actually being 3,150 litres).


From the 4 separate calculations made in the above it will be seen that increases of 25% + 10% + 30% + 5% (totalling 70%) must be applied. This percentage of the example 3,000 litre pond equates to an increase of 2,100 litres, and therefore for TRUE calculations purposes in selecting a suitable filter it must be capable of handling 5,100 litres (3,000 + 2,100).