Proper water flow is crucial for success with reef invertebrates. However, it is an often neglected and surprisingly inflexible parameter for coral health. Water movement is one of the three fundamentals in reef aquarium husbandry along with light and food. Fortunately, it is also the least controversial dynamic and a very straightforward and simple endeavor. While inadequate lighting of symbiotic corals can often be compensated with extra feeding, inadequate water flow is categorically limiting to growth and increases the risk of morbidity.
As predominantly sessile animals, reef invertebrates are critically dependent on specific water movements to not only help them thrive, but to survive in oceans and aquariums alike. Proper water flow serves many purposes including but not limited to the following: the carriage of food and other nutrients to a coral, the carriage of waste products away, the exchange and dispersion of sexual cues and gametes, purges suffocating detritus, disperses and dilutes familiar and alien allelopathic compounds, and stimulates and supports new growth.
It is very difficult to create too much water flow in most reef aquariums given the very dynamic environment that most corals hail from, but it is possible to dispense water flow improperly.
There are three basic types of water flow on a reef that we commonly replicate in aquaria: laminar, surge and random turbulent.
Laminar flow is the easiest flow to produce and the most commonly mis-applied in aquaria. Submersible pumps and powerheads produce laminar flow: a one-directional, linear movement of water. Among popular species of coral in the trade, few will tolerate long term exposure to this type of water flow and even fewer actually need it. It is plainly harmful for many species and is especially harsh on fleshy, large-polyped stony corals (LPS). For sessile animals ill-adapted to weather an imposed laminar flow, prolonged exposure can literally abrade or denude living tissue! However, there are reef invertebrates that do favor laminar flow and grow strategically to exploit it. Sea fans are the most conspicuous example of reef invertebrates that favor this type of water movement. The large, flat plane of a fan gorgonian's body grows perpendicular to the brisk path of water flow to fully exploit the uni-directional movement. Not all gorgonians favor brisk, laminar water movement, though; there are as many or more species of gorgonian that favor surging water movement. To re-iterate, only a minority of corals commonly kept in aquaria will thrive with laminar water movement. Most corals prefer random turbulent or surge water flow.
Surge is a dynamic form of water flow that is the hardest to replicate in aquaria and yet the most useful to employ categorically for the health and growth of reef invertebrates. It is characterized by a rush of water in one direction and then an equally voluminous change or exit of water in the opposite direction at timed intervals. Most corals tolerate or favor this type of water motion. Polyps and whole animals, if flexible, advance and retreat almost hypnotically in the surge having either side of their structure cleansed and bathed in the life supporting flow. Unfortunately, it is very inconvenient to produce surge flow in small home aquariums. Specialized aquariums and equipment are needed to produce and contain the wildly fluctuating volume of water in the system. Furthermore, the waves and "salt creep" produced by the delivery of water in this manner are more challenging issues to contend with than most home aquarists care to embrace. Display aquariums in living spaces are necessarily more tidy and compact. They often cannot afford the space or aesthetic compromise of a deeper vessel or overhead surge device. There is also the nearly inevitable concern about the sound ("noise" to some) of surge activity in aquariums within the confines of a living space. Ultimately, surge activity is remitted most only to larger aquariums and public displays where such special considerations can be addressed. Aquarists interested in creating surging water flow might begin by referencing The Carlson Surge Device or the Borneman Flush Device (page 341 of Borneman's indispensable work "Aquarium Corals"). For all other aquarists, rest assured that the creation of random turbulent water flow in aquaria instead is a small compromise that still has tremendous benefits to coral health for most species commonly kept.
Random Turbulent flow is perhaps the best universal type of water movement to create in aquaria for the optimum health and growth of reef corals. It is created by the simple convergence of laminar effluents to create a random and vigorous mix of water. Unfortunately there is no single recipe for creating this type of flow since we all rockscape our aquariums differently and such structural impediments change the dynamic of water moving through the system. Very basic experimentation by repositioning the nozzles or effluent stems of powerheads, pumps or returning manifold outputs will determine an arrangement that produces the best movement of water in the display. Ideal flow will stimulate coral polyps to extend eagerly and move about briskly at random. Also, it will prevent any unwanted "dead spots" where detritus could otherwise accumulate. Powerheads are the most commonly employed devices for moving water in aquaria. For those using powerheads, begin by positioning pumps to discharge water to the diametrically opposite end of the aquarium. A pump in the upper left portion of the display might be directed to the lower right area of the tank. Hopefully, another pump in the upper right portion of the tank can be employed to converge on this pattern. Additional powerheads used to disturb the balanced pair of pumps will likely be of great value. Some of the simplest and most effective pump arrangements I have seen have used the top-mounted Tunze "Turbelle" or Aquarium Products "Gemini" pumps in all four corners of the display directed at each other in opposition.
Ultimately, one of the very best and easiest ways to create random turbulent water flow involves a single large external pump (usually plumbed into a sump for the return of water) to reduce or avoid using unsightly pumps and powerheads in the display. A simple and solitary return line from the pump can sneak up the back of the aquarium to form a discreet closed loop of pipe that runs the perimeter of the display and is to be mounted just at or slightly above the running water level. This run of pipe will serve as a manifold to evenly distribute water flow to the tees plumped into the loop for a very fine-tuned delivery of water flow to the reef corals below. Small segments of interlocking, flexible pipe can be attached to the end of each tee for even more precise control. Else, a swiveling 45-degree elbow at the end of each tee can still deliver variable water flow nicely. Either way, a teed manifold will afford an aquarist nearly limitless opportunities to adjust the movement of water in the aquarium with ease as necessary.
Whichever form of pump and plumbing an aquarist chooses, flexibility of direction for water flow is critical to be able to continue to produce random turbulent movement in the tank. The rockscape might change (plants, rocks, etc.) and corals will certainly grow, and all such impediments will alter the dynamic of water movement in the display. Be assured, though, that a dynamic and random pattern of water flow is the least complicated and most effective way to provide adequate water movement for most popular corals.
Some Advice about Wave-makers and Wave-timers: in short… save your money and resist buying such toys! Although variable water flow is indeed better for most corals, it is not so critical that it warrants the complicated and wasteful employment of electronic wave creating devices. The interruption of water flow by timers and solenoid valves is really unnecessary and categorically less useful than the proper full-time employment of the unfettered pumps. Rest assured that the simple creation of random turbulent flow from converging energies will give you the most for your money! At the lowest common denominator, a wave-timer that shares the duty of any number of pump is only delivering a fraction of the same water flow (usually half, for staggered intervals) that could be otherwise be obtained by running the pumps in a dedicated fashion. Furthermore, the interruption of power (with or without soft-start features) places greater stress and wear on the pumps, and measurably shortens their lifespan.
Finally, in address of the specific amount of water flow needed for corals, we cannot only say that more is better. The type and volume of water flow in the aquarium must be tailored to suit each collection of corals that have hopefully been assembled with regard for their similar needs. The old "rule of thumb" for water movement was 4 to 10 times a tanks total volume. In modern aquariums however, a 10-fold turnover of water is mediocre at best. Now enlightened to avoid laminar flow for most corals, aquarists will find that 10 to 20-fold turnovers are common and appropriate. Some systems with various stony corals or programs targeting fast growth will likely employ even great flow. Judicious experimentation is the only rule to follow here.
And so, in this third installment we have completed an address of the three simple ingredients of light, food and water flow for the successful care and growth of corals. These elements are the foundation for a healthy aquarium and all certainly require manipulation and experimentation to satisfy the many different species we keep. Simply approach your aquarium husbandry with an open mind, consider new and old techniques alike, and make an informed decision that serves your corals based on an intelligent consensus. Reef keeping is still a pioneering endeavor. Embrace the pioneer spirit and carry on gently!
With kind regards, Anthony Calfo USA
Borneman, E. H., 2001. Aquarium Corals, Selection, Husbandry and Natural History, T.F.H. Publications, Neptune City, NJ. 464pp
Anthony Calfo is the author of the "Book of Coral Propagation", and co-author of the upcoming title "Reef Invertebrates" with Robert Fenner. He is a daily mentor and content provider for WetWebMedia.com and he can be reached for comment via e-mail at firstname.lastname@example.org