Water Flow in the Marine Aquarium
Using Powerheads to Create Flow

The following article is excerpted from Blue Zoo’s Director of Marine Ornamental Research Mark Martin’s The Complete Idiot’s Guide to the Saltwater Aquarium. 

Flow is critical to successfully keeping many saltwater species, especially sessile invertebrates. In fact, most serious reefers would agree flow is more important than light. The ocean is a dynamic environment where tides and currents are constantly moving massive volumes of water back and forth, up and down, in and out. Surface storms create chaotic wave action, while structures such as reefs cause their own internal waves and currents. Marine species have adapted to this environment to the point that many will only thrive if you replicate it in the aquarium.

Water movement is particularly important for an invertebrate or reef tank where many sessile invertebrates rely on the flow of water to bring them food and clean off waste. Water movement can also assist with the maintenance and health of a FOWLR tank, although it is not as critical for most fishes. While some biotope-specific tanks require minimal water movement, they are most certainly the exception to the rule.

Why Current Is Good

Current is essential to the feeding strategies of many sessile invertebrates. In short, it is the currents that carry the food to the animal, if the animal is unable to move to the food. Without current, many sessile invertebrates would simply starve to death.

Both in the wild and in the aquarium, strong flow is also critical for removing sedimentation—detritus and other particulate matter—that has settled on the substrate, rocks, and, worse, on corals themselves. Accumulated sediment on a coral can easily choke and even suffocate that coral. Conversely, strong flow constantly rids corals of this detritus, as well as the waste produced by the coral itself.

How Much Current Is Enough Current?

For many reef aquarists, water movement is said to be too strong only if the substrate itself is being blown into suspension and rocks are being moved by the current. Many an experienced aquarist might say that anything just shy of that is appropriate. We agree that it is hard to have too much flow in a marine aquarium; most water movement problems are the result of not enough flow instead of too much. Nonetheless, you can't simply add tons of flow and be done with it. There are many different types of flow, and too much flow of the wrong sort can easily kill a coral. In addition to creating flow, you need to create the right kind of flow.

Creating Alternating Currents with Pumps and Powerheads

While a closed loop system (CLS) with either a SCWD or an Oceans Motions unit is a good way to go when it comes to creating current in the aquarium, there are other effective alternatives that do not require a separate CLS (and the cost associated with a CLS). Current can be created, for example, using submersible pumps called powerheads.

Placing small powerheads in the tank itself can provide flow, but as you might imagine, a single powerhead mounted on the side wall of a tank will usually produce nothing more than laminar flow and, given the size of the pump, not very much of it. You could, of course, add a second powerhead and put it on the opposite wall of the tank so that the two currents meet in the middle. This is better, but it still only provides a limited space in the tank where there is variable flow. You could keep adding more powerheads at different angles, but now you are seriously cluttering the tank, probably increasing the temperature of the water, and increasing your operating costs due to power consumption. What to do?

The solution to this problem is to use fewer, more advanced powerheads and have them controlled by a dedicated controller (this is also sometimes called a wavemaker) or a master aquarium controller. By a "more advanced" powerhead we mean a powerhead that doesn't simply shoot a jet of high flow directly at a specific point. You want a powerhead that produces more marginal flow and that is even scalable in terms of its output based on how you set the controller.