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Moving water takes your pond to the next level.Recirculating and filtering water are not necessary in the “balanced” garden pond. If you do decide to move water, you will choose a pump size for one or both of two reasons:

For Aesthetics - To see and/or hear a waterfall, stream or fountain. As a general rule of thumb, move no more than twice the number of gallons in your pond per hour. This tends to keep the waterfall in scale, minimizes suspended particles, and probably leaves a quiet space for waterlilies.

For Filtration - To reduce algae, waste and decaying debris, and provide for higher fish stocking levels.
For filtration,
pump the number of gallons in your pond from once every four hours to twice an hour. Slower rates of flow allow bio-filters ample dwell time to work most efficiently.

Make your decisions in this order: What size is the pond? How many gallons of water does it hold? What do you want your moving water to look like? A Bubbling rock? Waterfall? Fountain? What would be the ideal rate of flow for the best filtration? Will compromise be necessary? Given all that, what size pump? Screen or mechanical filter? Hose? Bio-filter? Each decision is dependent on the previous one!

Determining Pump Size
When in doubt, spec. a pump more powerful than you think you need -- you can reduce the outflow, but never restrict the intake of your pump.

Determine the ideal flow rate:

Sill: The width (in inches) of the waterfall. For a sheer fall over a clean edge, allow 100-200 gph per inch of sill... less for a broken fall. This determines the "look".

Now – to select the appropriate pump, determine the following:

Static Head, or Rise: The vertical distance (in feet) from the surface of the lower pond to the outlet at the top of the waterfall or fountain.

Friction Head, or Run: As water flows through hose and fittings there is resistance. Determine the overall hose length (in feet). Maximize the hose diameter to minimize resistance. Consult a friction loss chart, or. . . For a simple guideline, Assume that 10 ft. of run equals 1 ft. of rise.

Pressure Head:  Additional pressure caused by filters, U.V. lights, etc.

Total Dynamic Head: The sum of the static head + friction head +  pressure head is equal to the total dynamic head (TDH). It represents the total amount of resistance your pump is working against.  With this figure and a set of pump performance charts you can determine which pump(s) will provide the desired flow rate for your project.

Other Considerations: There are usually several options for a pump in a given range. Initial purchase price, projected monthly electrical usage, length of electric cord, physical dimensions, performance, style of inlet/outlet & warranty period are all to be considered.


Mechanical Filtration: Removes particulates from the water. Usually positioned before the pump intake, e.g. skimmers & foam filter boxes, also preventing damage to the pump. The surface area and density of the materials in the filter directly affect the cleaning frequency.

Biological Filtration: Converts toxic ammonia and nitrites generated by fish waste and decaying debris to non-toxic nitrates, by passing water over filter media harboring nitrifying bacteria. The amount of biological activity taking place is directly related to the flow rate of water and the amount of surface area provided by the media.

Plant Filtration: Minimizes nitrate buildup in the pond by use of plants in the water garden and biofilter. Passing nutrient-rich water over, around and through the roots of plants causes a rapid uptake of nitrates and starves single-cell algae out of existance. Plants at the top of the biofilter help reduce pea-soup algae.

UV Sterilizers: Provide a sure cure for the green water blues! Water is pumped through a sealed chamber where harmful waterborne micro-organisms and free-floating algae are exposed to radiation emitted by a UV lamp. Proper flow rate is key to success.

A filter is key to constant pond visibility.

Drawing courtesy of Maryland Aquatic Nurseries, Inc.

Maintaining the Pump and Filter
Mechanical filters are designed to trap debris and must be cleaned out to remove that debris from the system. Service your mechanical filter regularly! The number 1 cause of pump failure is clogging the intake.

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