How To Properly Size A Return Pump For An Aquarium
Water flow plays a critical role in the health of your saltwater aquarium, especially if you are keeping corals. For fish, proper flow ensures gas exchange and plenty of oxygen and helps to move waste and nutrients into the filtration system for removal. With coral, that flow delivers food and critical nutrients as well as washes away waste. Aquarists provide flow in their aquarium using powerheads and a return pump.
While Powerheads are responsible for internal flow and creating currents inside the display, a return pump is what moves water through the filtration system. In the situation where you have a sump located down below the tank, your return pump moves water from the sump back up into the aquarium. In an All-In-One style tank, the return pump moves water through the back filtration chambers and into the display. In either case, the return pump is creating a constant, circular flow of water through the display aquarium and filtration system; exactly why we call it "the heart" of your aquarium.
Although it might seem a little confusing at first, choosing an appropriately sized return pump is pretty easy. With just a few simple calculations you can be confident your making the right choice.
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Gallons Per Hour (GPH)
Water pumps are rated in gallons per hour or GPH for short. The advertised GPH rating is the maximum flow rate for the pump which is the amount of water the pump can move when it isn’t connected to anything. Think of it as the flow rate with the pump sitting in a bucket of water. It’s the industry standard for measuring the power of a particular pump, but it doesn’t tell the whole story. Once you connect the pump to tubing or PVC pipe, it creates pressure (head pressure) and the flow rate will be reduced. You need to account for this loss of flow when choosing your return pump.
Calculate Your Recommended Flow Rate
For reef aquariums, the general rule among hobbyists is to find a pump that can deliver at least 5x to 10x the tank's total water volume per hour. For example, for a 50-gallon tank look for a pump rated at least 250-500 GPH. When the pump is in use on your tank, the head pressure created by the plumbing will slow the pump down and likely achieve a flow rate somewhere in the 150 - 350 GPH range, right in the sweet spot.
While 10x total tank turnover per hour is pretty much the absolute maximum flow through an appropriate sump or filtration system, hobbyists using modern filtration techniques often choose to run a little slower in the 2x to 5x total tank turnover per hour after head pressure is applied. This will increase dwell time in your sump and run a little quieter as well.
Example: 50 gallon tank = Actual flow rate of 100 - 250 GPH
No matter your preference for slow or fast flow, always use your tank's total water volume to estimate how many times per hour the return pump can cycle your entire water volume through the filtration system and know that anywhere from 2x - 10x total tank turnover per hour is generally acceptable.
When choosing a pump, it's important to consider the amount of head pressure caused by your particular plumbing to accurately gauge the pump's capability. You can calculate head pressure using standard formulas (described below) and you always want to have some kind of flow control over your pump whether that be an inline valve or the use of a DC controllable pump. You can always reduce the flow with a valve but you can't make the pump go faster.
It should be noted that a tank's drain will have a maximum rate of flow as well as a sump which means only so much water can pass through per hour. Most overflow boxes and some sumps advertise maximum flow ratings which come in handy when deciding the appropriate drain and sump for your tank. It is a pretty rare occurrence that your return flow surpasses your maximum drain/sump flow rate as long as you do not exceed 10x turnover per hour but nonetheless, all the more reason you need to have control over your return flow and size your return pump appropriately.
Pumping water up to the aquarium is hard work. Gravity and friction inside the pipe, tubing, and valves reduce the flow rate. The higher and farther the pump has to push the water, the harder it is for the pump to move the water which ultimately slows down the flow. This resistance is called “head pressure.” The higher the lift and longer the distance, the greater the resistance to flow.
If you ultimately need a flow rate of 100 GPH and selected a pump that produces a maximum flow rate of 100 GPH, you would be very disappointed after installing it on your tank. The tubing, elbows, and head pressure would dramatically reduce the actual flow rate. You need to choose a pump that is stronger and can deliver the flow you need AFTER the head pressure is applied.
In order to calculate head pressure, use the following standard formulas to add up the vertical rise, distance, and 90° turns to come up with a head pressure number.
- Every 1 ft of vertical rise = 1 ft of head pressure
- Every 90° elbow fitting = 1 ft of head pressure
- Every 45° elbow fitting = 0.5 ft of head pressure
- Every 10ft of flat horizontal distance = 1 ft of head pressure
Sizing Your Pump
You can use a pump manufacturer's flow chart to determine the true pump flow rate based on how much head pressure is being created. Pump manufacturers supply these charts or graphs as a starting point in determining how to size a return pump for your aquarium.
The idea is the choose a pump that is powerful enough to deliver the recommended flow rate you calculated earlier, at the given head pressure your plumbing creates. In the example below, the water rises roughly 4ft vertically, 4ft horizontally, and then passes through two 90° elbows. That would calculate to be roughly 6.4 ft of head pressure.
(4ft vertical lift = 4 ft of head pressure) + (4ft horizontal = 0.4 ft of head pressure) + (x2 90° elbows = 2 ft of head pressure) = 6.4 ft of total head pressure
Using the flow chart above modeled after the Neptune System COR-15 pump, we can estimate approximately 700 GPH of flow at the given 6.4 ft of head pressure. These are only estimates but this simple calculation gets you in the ballpark in terms of the actual flow rate on your particular tank.
Final Thoughts and Tips
It’s important to remember that you should always choose a slightly more powerful return pump rather than a questionably undersized one. This is because you can always dial the flow rate back with a valve or controller, but can’t increase the flow rate if the pump is undersized. Having a little extra pump power is never a bad thing either because this means that you can add reactors and UV sterilizers down the road using the return pump without taking away from your display tank's flow. Just be sure the pump will physically fit into your particular sump or filtration compartment.
AC pumps run at a constant speed and should always be plumbed using an in-line valve with a true union connection between the pump and the tank, this allows you to not only control the flow but also completely close the plumbing line when you turn off the pump for maintenance. You can also easily remove the pump using the true union connection without having to disassemble all of the plumbing.
DC-powered controllable pumps come with electronic controllers that allow you to change the pump speed and resulting flow rate which can be a great benefit when dialing in your tank's overall water flow. They also run quieter and are a bit more efficient in terms of electricity consumption. It is still a good idea to have an inline valve with union plumbed with a DC pump, but not absolutely necessary to control the flow.
**Pro Tip: If you want to get the most out of your pump, do not restrict the diameter of the plumbing. If the plumbed tubing is smaller than the inlet and/or outlet of your pump, it restricts the flow and slows it down. It’s best to use the same inlet and outlet diameter plumbing or whatever is recommended by the manufacturer for optimal performance and longevity. Also, be sure to keep your pumps clean; letting a pump run with calcareous build-up will drastically reduce the performance and overall lifespan of the pump.