Calculating Head Pressure

Before purchasing a return pump for your saltwater aquarium’s open system, you will need to calculate both the head pressure on your system’s return line and the flow rate (in gallons per hour or GPH) you desire inside your tank. Here is how to do those calculations and then use those calculations to choose an appropriate return pump based on a pump manufacturer's flow curve chart.

Calculate the Head Pressure

To determine the estimated expected head pressure on your aquarium’s return line, follow these steps:

1) Measure the lift (the distance between the return pump and the level of the return outlet(s) in the tank.

2) If the tank has multiple returns, measure the length of any horizontal plumbing to the lift.

3) Calculate the resistance caused by the return line’s plumbing configuration by adding 1.5 feet for every 90-degree turn (although you’ll need to add up to two feet if the return line diameter is small or as little as one foot if it is large).

4) Calculate the back-pressure caused by submersed returns in the tank by adding an additional foot for each return that is not more than several inches deep (all your "open system" returns should be mounted high in the tank to avoid flooding in the event of a power outage).

Add all of this together, and you should have a rough idea of the head pressure your return pump will be facing.

Calculate the Flow Rate

To determine flow rate, multiply your complete system volume by the number of times you want to completely turn over your system volume per hour. This will give you your desired flow rate in gallons per hour or GPH.

Plan on turning over the complete volume of the system no less than seven times per hour. For a reef tank, increase the flow rate to at least 10 times per hour up to as much as 30 times per hour depending on what species and what biotope or ecosystem you are attmpting to replicate.

NOTE: Remember that your return pump need not be responsible for all of the tank’s overall target flow rate. Many aquarists supplement their return pump’s flow rate with a closed loop system (CLS) or individual powerheads mounted in the tank.

Consult Flow Curve Chart

Once you have calculated the estimated head pressure and desired flow rate, consult pump manufacturer's flow curve charts to determine what pump is best for your system.

Additional Resources

• Bob Fenner's "Plumbing Marine Aquariums"
• "An Engineering View of Aquarium Systems Design: Pumps and Plumbing"

A Sample Flow Curve Chart from a Sequence Pump

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