Water Gardeners International
Home | Join WGI | Members' Exclusive | Gateway to Water Gardening

 For Beginners

Water Garden and Pond

by Joel Police
New Haven, Indiana USA

Author’s Note: These articles are intended as general reference only. The information presented represents my perspective gained from experience as a water gardener and as a business owner. The views expressed here are also influenced by the conditions found in the Midwest and may not be as applicable in other geographic regions.

Safety Caution: All discussion of pumps assumes that the pump is plugged into a ground fault interrupter circuit (GFIC). In addition, a pump with a cut or damaged cord should never be repaired and put back into service by a homeowner. Always unplug the pump before working in a pond, especially if the work requires physically entering the pond.

Water pumps function as the "heart" of any artificially created aquatic ecosystem. Pumps supply the water movement necessary to foster oxygen exchange via waterfalls, streams and fountains in addition to driving the filtration system vital to fish health. Pumps enable the sound and visual interest so closely associated with our peaceful and tranquil hobby. While few debate the importance of pumps to water gardens, views differ concerning the type, size, manufacturer and design of the pump.

When researching pumps, first consider its flow rate, the amount of water a given pump can "move" during a given time. Commonly measured in terms of either gallons (1 gallon = 3.8 liters) per minute (GPM) or gallons per hour (GPH), the flow rate describes the amount of work a pump performs. Some pump manufacturers also include a horsepower rating. However, pay more attention to flow than to horsepower since many design factors influence how much horsepower is needed to produce a given flow rate.

Manufacturers offer flow rates from a few gallons per hour to thousands of gallons per minute; a pump exists for most any situation. However, before deciding on what size pump to purchase, first identify what optimal flow rate you wish to achieve. A filtration system, waterfall, stream, pond size and the number of fish all impact pump selection. Clearly define these parameters during the planning phase to the pump choice much easier.

To illustrate why flow rate is not the sole parameter used for pump selection, consider this example. Assume you are building a 1000 gallon (3784 liter) water garden with a box filter/skimmer filtration system and a 2 foot (.6 meter) tall waterfall. You also want a few goldfish in your pond along with waterlilies since you have a nice sunny location. With this in mind, what flow rate pump should you choose? A 1000 GPH (3784 LPH) pump? A 2000 GPH (7569 LPH) pump? Perhaps a 3000 GPH (11353 LPH) pump?

Since the rule of thumb for small ponds is to turn over the water twice an hour to ensure proper circulation and help moderate water temperatures, the apparent answer to the pump size question would be a 2000 GPH (7569 liter) pump (pond size of 1000 gallons (3784 liters) x 2 turns per hour = 2000 GPH (7569 LPH). Yet before you rush to purchase a pump, consider the rest of the example pond. The filtration system, waterfall and plumbing components utilized to make everything operate properly also consume a portion of the pump's flow. What causes this?

The simple answer is resistance or friction. Just as wind resistance is explained in terms of drag, water resistance (in plumbing) is described in terms of head. In this example, you need a pump strong enough to circulate the water twice hourly, power a filtration system and run a waterfall. All these actions place a demand on the pump to lift the water, force it through tubing and work against the restrictions of a filtration system. To accomplish this work, friction (head) results, meaning the pump does not produce its optimal rated output.

A pump rated at 1000 GPH (3784 LPH) can only produce that flow without added resistance. Water gardens present compromises that influence the amount of resistance in their plumbing systems. Whenever you use a smaller pipe diameter to save a little money, the head increases. Need an extra elbow to squeeze your filter into a tight spot? Then figure in a little more head resistance to the system. Want that waterfall to be three feet tall instead of two? You guessed it...more head pressure.

The two basic principles of water resistance acting against your pump are friction loss and static head. Any given pipe, plumbing fitting, filter or opening water moves through or across creates drag. Frictional loss measures the amount of water flow rate lost to drag. Static head refers to the distance between the pond water surface and the highest point to which the water is lifted. By adding those figures together, you have the total dynamic head (static head + frictional loss = total dynamic head) stated in feet or meters.

The existence of total dynamic head directs you to select a pump with a flow rate greater than our desired turnover rate. If we had chosen a 2000 GPH (7569 LPH) pump for our example pond (matching our turnover rate), the result would be a significantly reduced turnover rate. What makes total dynamic head so difficult to appreciate is its unseen effects. However, if you fail to account for it, you run the risk of seriously under-sizing the pump and producing troublesome situations due to insufficient flow.

The good news about total dynamic head is a wealth of information exists to help you calculate and account for it. Numerous charts and graphs in water gardening books and on the internet provide friction loss figures for tubing and fittings (one example below). Filter manufacturers rate their equipment based on pump flow rates to aid your decision-making. Use a larger size than your calculations suggest. You can restrict an excess flow, if desired, but you cannot make a pump deliver a higher flow rate than its capacity.

Once you determine your desired turnover rate and calculate the number of feet or meters of dynamic head in your planned system, you are ready to look at pumps. Upon examining the technical data presented with the pump, you will notice a clearly stated flow rate, often accompanied by the horsepower rating and an electrical consumption figure. You should also see a performance chart or performance curve on the box or included with the literature. Avoid any pump without this performance information, a crucial guide in pump selection. More below . . .

This performance flow chart shows the relationship between volume (measured in GPH or LPH) and head (measured in feet or meters). Many charts include the shutoff value - the pump's maximum dynamic head. For example, in Figure 1 the red line represents a pump rated at 500 GPH (1892 LPH) at 0 feet of head. As the head increases, the flow rate decreases. The shutoff value occurs at 12 feet (3.7 meters) of head, meaning the pump essentially stops moving water with that amount of head.


Let's go back to the example pond from earlier in the article. Assuming a volume of 1000 gallons (3784 liters), a static head (waterfall height) of 2 feet (.6 meters) and frictional loss due to tubing, fittings and filtration equipment of 8 feet (2.4 meters) of head, you have 10 feet (3 meters) of dynamic head. The 2400 GPH (9082 LPH) pump (green) might seem like a good choice for your pond. However, its performance curve shows that the 2400 GPH (9082 LPH) pump only moves roughly 1300 GPH (4920 LPH) at 10 feet (3 liters) of head. Furthermore, the 3600 GPH (13623 LPH) pump (blue) produces a flow of approximately 2500 GPH (9461 LPH) at 10 feet (3 meters) of head.

Armed with this information it becomes clear that the 2400 GPH (9082 LPH) pump fails to produce the desired 2000 GPH (7569 LPH) flow. The 3600 GPH (13623 LPH) pump produces more than your requisite 2000 GPH (7569 LPH), so it becomes a candidate. Remember that this chart illustrates the performance data for just one pump manufacturer, highlighting the importance of this data for any pump you consider. Often, pumps from Manufacturer X display a significantly different performance curve than those from Manufacturer Y.

Shopping online or at water garden supply stores, you find a tremendous selection available. Besides flow rate, you must choose between a 110 or 220 volt model, an external or submersible installation location and direct drive or magnetic (mag) drive propulsion.

Picking the pump voltage is a good starting point. While you might already have an outdoor 110 volt circuit, it may be advisable to upgrade to 220 volts. The 110 volt pumps offer the best combination of price, operating cost and ease of installation for low to medium flow (100-7500 GPH) installations while 220 volt models excel with high flow rates or when a situation would otherwise call for multiple 110 volt pumps. Speak with a licensed electrician about what type of circuit you have or can install for your feature.

Another key pump consideration is its location. As the name implies, external pumps operate outside of the physical confines of the water garden. They typically appear in large water features or with higher end koi ponds and are making inroads into traditional EPDM liner ponds. Quite often external pumps (fitted with basket strainers to capture large debris) connect directly to bottom drains and feed vortex filters or settling chambers as they move water to the filtration system. Some skimmer designs include provisions for integrating external pumps.

One reason external pumps are growing in popularity is easy access for maintenance. Being outside of the pond greatly simplifies cleaning and removing debris. For physically challenged individuals this fact alone makes external pumps attractive. In addition, because external pumps consist of a separate power unit and propulsion unit, manufacturers can "custom make" a pump to fit your needs. The power unit may work with either a 110- or 220-volt circuit. Diverse impeller designs enable a performance curve closely matching your desired flow rate. From a safety standpoint, external pumps greatly reduce the risk of electrical shock to pond inhabitants.

Nevertheless, external pumps do have some disadvantages. External pumps frequently cost more than a submersible pump with the same flow rate. In order to achieve the operating savings they offer you need an electrician to install a 220-volt circuit to your pond area. Hiding and housing the pump in a pond setting can be challenging. Noise can be a downside to external pumps. External pumps may need priming before use, which is problematic for those who frequently shut down the pump. Finally, in cold climates, winter operation is troublesome because their exposed plumbing is subject to freeze damage.

While external pumps continue to gain acceptance, the great majority of water garden pumps are the submersible type. Don't confuse submersible water garden pumps with sump pumps. True submersible water garden pumps are engineered for the rigors of the pond environment and include safety features to protect humans and wildlife. Submersible pumps sit on the pond floor or in a skimmer.

The chief advantage of submersibles centers on convenience. Submersibles are easy to plumb, require no special wiring and need little care other than cleaning, making them especially friendly to new water gardeners. Furthermore, submersibles usually cost less than a comparable flow rated external pump and are widely available at water garden supply stores.

Clogging is their chief drawback. Being in the pond, they risk sucking up leaves, twigs, stones or other debris. Although easily remedied, a clog may lead to pump damage or even failure if not cleared in a timely manner. Housing the pump in a skimmer is not always foolproof. The skimmer basket or net can clog, cutting off the water supply to the pump. Wildlife (usually frogs or toads) can block the pump inlet and stop the pump. The life span of submersible pumps is sometimes an issue, especially with the cheapest models. The 220 volt models yield a longer life than 110 volt models. On average, external pumps tend to last longer.

Classification of pumps can further be based on the method used to propel water. Direct drive and magnetic drive pumps reflect different engineering approaches to pumping water. Direct drive refers to a copper-wound armature that spins when current is applied, and rotates a shaft and the impeller. Water is drawn in, and then expelled through an output port to the filter or waterfall. Direct drive pumps mirror the technology found in most common electrical devices like vacuums, blenders and hair dryers.

The more popular type, direct drive pumps are relatively inexpensive to produce. They provide a good flow-to-operating-cost ratio and their performance curves usually best those of magnetic drive pumps. Because the technology has been around for so long, direct drive pumps have numerous adaptations for almost any water gardening chore. As you research pumps, notice that the biggest and most powerful pumps in the market are direct drive units. Manufacturers continually improve performance, reliability and efficiency.

The main enemy of direct drive pumps (external and submersible) is heat. Since the units contain brushes, bearings and seals, the potential for failure greatly increases when the pumps operate at high temperatures. Most often, pumps fail due to running dry (low water situation), cavitation (air bubbles in the water), or running too close to their shutoff limit (too small a pump). Even though pump failure is commonly due to a small internal component, replacement is more cost effective than paying for repairs. While the technology is tried and tested, many argue that direct drives have reached a point of diminishing returns and in an environment of increasing energy costs, they are simply becoming too expensive to operate.

This desire to lower operating costs led to additional research and the design of magnetic drive pumps, or mag drives. Mag drives consist of an epoxy sealed electromagnetic coil with a hollow central cavity. A magnet with an attached impeller shaft sits in the cavity and spins when current is applied. Like direct drive pumps, water is pulled in through the inlet and expelled through an outlet port.

Since mag drives use no bearings, seals or brushes, they have a greatly reduced failure rate due to wear and friction. The impeller assembly is the only moving part and can easily be replaced if damaged. Because of design differences, mag drives are more compact than direct drives. They dominate the market for low flow pumps, being especially well suited for statuary and tabletop water features. Of course lower energy consumption is their claim to fame; mag drive manufacturers claim operational costs about half that of a comparable flow direct drive pump.

However, the main knock against mag drives centers around lack of power and poor performance in higher head applications. The pumps actually perform quite well, but installers must pay close attention to the performance charts to make sure the pump is right for the job. As mentioned, mag drives typically flow lower volumes as head increases compared to a direct drive unit. Currently mag drives are only offered up to 10000-GPH (37843-LPH) flow rates, making a limited selection.

Another complaint about mag drives involves impeller damage from debris. Whereas most direct drive pumps easily handle sucking up leaves, twigs and even small stones, mag drive impellers may be jammed or broken by such debris. Many mag drives come equipped with foam pre filters, but these are commonly discarded due to a tendency to clog and required frequent cleaning.

Given today's wide choices of pumps and a well thought out design and construction plan, you will find a pump suited to your needs. Keep in mind that pump technology is constantly evolving. Even as mag drive pumps gain in popularity, new products are on the horizon.

One of the most promising pumps is an adjustable flow pump. These innovative programmable units enable you to adjust flow based on the time of day, the season of the year or even during feeding times. Not only does this save on operational costs, it opens up a completely new realm of design possibilities for creative water gardeners. Innovations are essential to our hobby, especially in light of the ever-growing attention paid to conservation of our natural resources. Water gardens reflect our attempt to recreate nature; anything we can do to preserve the environment is positive.

As a final thought on pumps, seek the best quality pump you can find. Price is not always the best indicator of pump quality, so wade through the marketing claims and do your research. The warranty period, availability of replacement parts and recommendations from other water gardeners should influence your choice. Avoid installers who use a certain brand pump simply because it comes with the "kit" or "system" they install. Nothing ruins the enjoyment of a water garden or water feature quicker than a failing pump, even if it is covered by a warranty.

I have always been a strong proponent of doing business with a local water garden supply store; regarding pumps, I cannot stress this enough. Those bargain pumps at the big box store may be priced right but are certainly not a good value in the end. I have witnessed piles and piles of failed pumps from a well-known and "reputable" aquascaping supply company that supplies installers all over the country.

Why do unsuitable pumps keep finding their way into our water gardens? Simply due to our own lack of diligence and willingness to seek out the information readily available to guide our purchasing decision.

Introduction to this multi-part series
Your Water Garden

< Water Features, Water Gardens, and Specialized Ponds
< Planning: Location, Design, Action
< Materials and Components for your Pond
< Filtration Systems for Water Gardens and Koi Ponds
Lighting >

Join! It's free!

New & News

 Water Gardening




Aquatic & Marginal Plants

WGI Online Journal

 Events Calendar


Botanical Gardens

Truly Named wgi©


Pond Fish


For Beginners

 Newsletter Archive


Water Garden Books

Pond Safety

 Members' Exclusive


WGI Stuff

About WGI

Home | Site Map | Gateway to Water Gardening

 Water Gardeners International
2211 S. Atlantic Av
Cocoa Beach FL 32931 USA