Choosing a Return Pump
Understand Head Pressure Before Plumbing Your Aquarium
An aquarium's flow rate is influenced by how many gallons per hour of water are pumped back to the tank by the return pump. Here is how to calculate what pump you need.
A marine tank’s circulation is like the body’s circulatory system. It is essential to keep water moving through the system, just as it is important to keep blood flowing through the body. Like the body’s circulatory system, failure to move water at the proper rate can result in poor system health and even death. In the body, the heart is responsible for pumping the blood. In the aquarium, it is the return pump. It’s necessary to calculate how much work this pump will be required to do based on the type and configuration of the plumbing. This article looks at how to calculate necessary return pump back-pressure or head pressure so that the appropriate circulation can be achieved.
In the sample system discussed in this series–a 135-gallon tank connected to a sump and several biotope-specific tanks–the water flows to the sump via an overflow and is then pumped back to the tank via a return pump. This is known as an "open system." It relies on two primary mechanisms to keep the water flowing: 1) gravity moves the water from the tank to the sump, and 2) the return pump pumps the water back to the tank.
As previously explained, the return pump must be matched to the size of the display tank’s overflow. A 1.5-inch standpipe was chosen for the 135-gallon tank so the system could handle a return pump capable of completely cycling more than 10 times per hour (once the closed loop system (CLS) is factored in, the total turnover in the tank will be 20+ times per hour).
One major factor complicates the process of choosing an appropriate return pump based on its published GPH rating–resistence in the return line. To return to the human body analogy, if the return pump is like the heart, then the plumbing is like the veins. The human body’s circulatory system relies on the veins being free of obstructions in order to function efficiently. The human body can regulate the size of the veins to provide either more or less blood to parts of the body, but in the aquarium, the diameter of plumbing chosen for the system is fixed (although it can be regulated with a ball valve). This diameter is a significant factor in determining how much back-pressure the return pump will experience.
In addition to diameter, every angle in the return plumbing causes additional resistence and increased back-pressure. As a result, the diameter and configuration of your return line has a large impact on how efficiently your return pump will operate. It is therefore essential to calculate anticipated back-pressure (generally referred to as "head pressure" by pump manufacturers) in order to decide what pump you need.
The 135-gallon system has roughly 21-feet of head pressure. The goal is to turn over the water 10 times per hour (about 2300 GPH--the entire system holds ~230 gallons). Knowing these figures, the pump manufacture’s flow curve chart can be consulted to determine which pump provides the desired GPH.
The 135-gallon tank uses a Sequence ReeFlo Hammerhead as its main return pump. While that pump may appear to be overkill (it is rated at 5800 GPH), the chart shows that at twenty feet of head pressure, it will provide about a 2400 GPH flow rate, which is about perfect for this system.
Additional Resources:
- Discuss Article.
- Dr. John Blatchford's articles on the topic.
- This is Part 5 in the Biotope-Based Marine Aquaria series. Read Part 4.
