How To Size a Return Pump

Water flow is critical for your reef’s health. Corals and other marine life can thrive only if they’re receiving the right water movement. On a reef, waves and tides constantly flush away waste products and bring in nutrients and suspend plankton. The same is true in our reef aquariums. A return pump moves water through the filter sump and back into the aquarium, creating constant water movement in the tank. One of the most common questions we receive is about matching the return pump to the aquarium. There are so many to choose from that it can get a little confusing. But no worries! Here’s what you need to know about choosing the right return pump for your reef tank.

Calculate Your Recommended Flow Rate

For reef aquariums the general rule among hobbyists is a turnover/flow rate of 5 to 10 times the tank volume per hour. For example: a return pump for a 50-gallon tank would equal a flow rate of 250-500 GPH (at the point of returning to the display tank).

equation to calculate the right size pump for your tank

Gallons Per Hour (GPH)

Water pumps are rated in gallons per hour or GPH for short. You’ll often see pumps rated at a certain GPH rate. Keep in mind that’s probably the maximum flow rate for the pump, but not in a real-world situation. The maximum flow rate is the amount of water the pump can move when it isn’t connected to anything. Think of it as flow rate with the pump sitting in a bucket of water. It’s the industry standard for marketing purposes, but it doesn’t tell the whole story.

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Head Pressure

Plumbing depiction of head pressure that will influence the decision of which pump size is needed

Pumping water up to the aquarium is hard work. Gravity and friction inside the pipe, tubing and valves reduce the flow rate. The higher the pump has to push the water, the lower the flow rate. This resistance is called “head pressure.” The higher the lift, the greater the resistance to flow. If you wanted a flow rate of 100 GPH and selected a pump that produces a maximum flow rate of 100 GPH, you would be very disappointed. The tubing, elbows, and head pressure would dramatically reduce the actual flow rate in your aquarium. That’s why pump manufacturers provide flow charts based on lift height.

You can determine on a pump manufacturer’s flow chart the true pump flow rate based on how high the water is being pushed up to the tank. Pump manufacturers supply these charts or graphs as a starting point in determining how to size a return pump for your aquarium. But there’s more to consider. Every 90-degree fitting adds the equivalent of 1 foot of height. Horizontal plumbing also adds 1 foot of head pressure every 10 feet, and vertical plumbing is 1 foot to 1 foot of head pressure.

Sizing Your Pump

example of a plumbed tank to calculate head pressure in order to size a return pump
Example of calculating head pressure for your aquarium, from sump to return.

As an example, let’s use the 50-gallon tank plumbing mock-up to calculate the total head pressure, seen above. Plumbing the tank there’s about 5 feet of vertical lift, five 90-degree fittings and 1.5 feet of horizontal tubing. Totals calculates to approximately 10.15 feet of head. Now let’s look at the pump flow chart below. We can see that for a flow rate of 250-500 GPH we could use the Sunpole Magnus VSG-6000 DC Controllable Return Pump. At approximately 10 feet of head pressure it will pump about 500 GPH, within the recommended range of flow.

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Charted example of a pump's flow rate in correlation to head pressure
Estimated flow rates

Final Thoughts and Tips

It’s important to remember that if you choose a more powerful return pump you can dial the flow rate back with a valve. DC pumps have the benefit of using an electronic controller to regulate the flow rate, but can’t increase the flow if it is an undersized pump. That’s why we recommend going with a slightly more powerful pump for your reef tank, as it can be dialed back, and will allow the option of plumbing media reactors or a UV sterilizer to it in the future.

If you want to get the most out of your pump, keep in mind mismatched tubing will reduce the pump’s output. If the plumbed tubing is smaller than the inlet and/or outlet, it restricts the flow rate of the intake and outlet of the pump. It’s best to use the same inlet and outlet diameter as recommended by the manufacturer.

And now that you know how to size a return pump, you’ll find it much easier to select the best return pump for your aquarium.

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