If you spend much time reading the online forums you’ll know there’s a lof discussion regarding powerheads and other flow devices. A great deal of time is spent trying to weigh the relative merits of one style of pump over another. This is no small task. When a flow device is submerged and powered on, it moves water. But in moving water it opens the doors to one of the most complex fields of physics. This is the field of fluid dynamics.
Fluid dynamics is normally something we can leave to the people with lab coats and multiple degrees. But then, reef aquarium enthusiasts aren’t know for shying away from technical matters. The reef aquarium hobby is probably one of the most cross-disciplinary hobbies there is. It involves elements of chemistry, biology and physics, and it involves all of these on a more than superficial level. Many long-time hobbyists point to this as one of the reasons the hobby has held their attention so long. While the chemistry and biology sides receive the most attention, the physics of moving water is also frequently discussed.
Yet one of the difficulties encountered when laypeople need to discuss technical matters is finding the right words to use. Formal science goes to great lengths to define terms with exacting specificity. The lengths forum members go to in defining terms can be notably shorter. Nevertheless with enough patience and questioning, even the most untechnical person can learn a lot about the science of saltwater aquariums and find ways to use this knowledge to directly improve the health of the aquarium inhabitants.
The term we’re looking at now is ‘undertow’. It’s a term that comes up fairly regularly and it’s almost always surrounded by a small cloud of vagueness. This vagueness of terminology is a problem. If we’re not exactly sure how we define it, or to what we attribute it, then we’re not really able to assess its value. To that end we can simplify the discussion by first agreeing that undertow in the ocean is something quite a bit different. While the ocean’s undertow is a function of tidal and wave action, things for which we do have corollaries in the aquarium, it’s also greatly affected by topography, which is far less relevant to the aquarium setting.
Undertow in an aquarium can then generally be defined as a laminar movement of water. It’s the movement of water at the lowest strata, just above the sandbed, moving towards whichever flow device is powering it. So how does a flow device create undertow in an aquarium? Remember that any pump that pushes water, also pulls water. As water is ejected from the front of the pump, an equal amount of water must be pulled in from the sides to replace it. (It’s easy to see why this must be true; if it wasn’t, then a pump would push away all the water around it and be left sitting in a hole of air in the aquarium.)
Now when a pump is pushing a relatively wide stream of water away, then there’s less room for the returning stream of water to return. And as with so many things in nature, the returning water follows the path of least resistance to get back to the pump. When a wide flow pump is placed towards the top of an aquarium, as they generally are, and the entire top section of the water column is moving away in one direction, then that path of least resistance is down low, right above the sandbed. It’s ‘easiest’ for the water to return in that lowest layer, so that’s what it does. That is what creates the undertow.
What we can learn from this is that undertow is not intrinsic to any particular pump design. It’s a function of many factors: the flow rate and stream shape of the powerhead; the powerhead’s location within the tank; the size and shape of the aquarium itself; and the arrangement of structures within the aquarium. It isn’t really something that the powerhead alone can take credit for, though powerheads with wide flow do make it easier to create an undertow that’s easy to notice. Viewed in this light, an undertow is more of a side effect than a goal.
By identifying the conditions that lead to a clear undertow we can begin to determine if the benefits are worth the costs. Is it more important to have an undertow, or would aquarists be better off focusing on the quality of the flow in the upper strata of the aquarium where, after all, most of the flow-loving corals reside? This line of questioning gets us closer to our goal of correctly prioritizing the merits of different flow devices and our strategies for their placement and use inside the aquarium. It’s a topic that deserves more discussion and understanding.