Larry Lunsford

There are many types of pumps available. Most of the pumps on Koi ponds are impeller type (also called centrifugal pumps) Impeller pumps use a spinning impeller to move water. There are many impeller designs, but they all work on the same principle. The spinning impeller forces water out, away from the center, creating an area of high pressure at the outside and low pressure at the center. The housing around the impeller has the water intake over the center of the impeller and the water output at the outside edge of the impeller.

For Koi ponds, the important pump features to consider are: in-line or submerged, flow rate vs pressure, magnetic drive / shaft drive, power consumption, cost, durability, plus some misc. details.

In-Line / Submerged: A submerged pump is one that must be placed in the water to operate. Submerged pumps need to be under water not only to draw water into the pump, but many need the water around the pump to keep them from overheating and burning out. A point of concern for submerged pumps is that they are more likely to cause electrocution hazards simply because the pump and power cord are in the pond. In-line pumps are run outside the pond and have input and output connections. Most in-line pumps suited for ponds require a flooded input. This means that the water must gravity feed to the input since the pump can't generate suction. Some pumps are capable of being used either in-line or submerged.

Flow / Pressure: The output of a pump decreases as the pressure it must generate increases. Any good pump will have a spec. sheet that shows the flow vs pressure for the pump. Flow is typically quoted in gallons per minute (GPM) or gallons per hour (GPH). Pressure is generally quoted in feet of head pressure (feet, ft.) or pounds per square inch (PSI) (one PSI = 2.2ft). When selecting a pump, pick one that can generate at least as much flow as you need at the pressure you need. You can always reduce the flow with a valve on the output (don't try to control flow by restricting the input). The charts below show flow vs pressure for some of the more popular pumps. For most pumps, the flow decreases rapidly as you approach its maximum pressure. Do not operate pumps too close to their maximum pressure since small increases in pressure (such as filters becoming dirty) will cause large decreases in flow.

Magnetic Drive / Shaft Drive: A shaft drive pump uses a normal electric motor with coils, brushes, etc. and a drive shaft to spin an impeller. A magnetic drive pump has a shaft made of magnets with an impeller on the end. Magnetic drive pumps don't have brushes (the part that makes electrical contact to a spinning shaft) so they typically generate less heat and less electrical noise and require less maintenance.

Power Consumption: Power use should be one of the main criteria you use when selecting a pump. Many seemingly cheap pumps will end up costing you much more in electricity consumption than you save on their purchase price. A good example of cheap to buy but costly to own pumps are sump pumps. Sump pumps are relatively cheap, but in just a few months you'll more than pay the amount saved on your extra high electric bill.

For many pumps, the power consumed decreases as the pump pressure increases. Some manufacturers spec. their pumps by starting current, some by minimum current, some by average current. Take test method into consideration when comparing pump performance. Also consider the actual flow and pressure requirements of your pond. Most pumps actually consume less electricity when operated at higher pressures.

Cost: Something that every Koi keeper seems concerned about is cost. In general, you get what you pay for. More efficient pumps usually cost more. In order to save money, some pumps don't include a power cord.

Misc.: Here's a few more points to consider when choosing a pump. Some pumps are oil filled (typically sump pumps and other submerged pumps). An oil filled pump can get a leak and the oil can kill all your Koi - avoid oil filled pumps. If you're not sure whether a pump has oil, rock the pump back and forth and listen for sloshing fluid. Does the pump include a power cord? Is the cord long enough? Does the pump include a leaf basket? Does the pump run on 120VAC or 220VAC or either?