Gyre Tanks Figure 2 This horizontal gyre tank produced flow speeds between 15-22cm/s.
A gyre tank encourages the maximum amount of water motion momentum because it contains a divider to essentially turn the tank into a circuit. This specialized aquarium constricts the cross section of the water’s path so that all of the water is evenly moving in the same direction. A setup like this mostly dispenses with rock or other ornaments on one face of the divider so it reduces friction with the usual aquarium reef structure for at least one side of the flow’s fetch. The divider stretches nearly the entire length of the aquarium and it can be placed either horizontally or vertically. In a vertical setup, the divider rests on the middle of the bottom of the aquarium and it projects out of the water surface. In a horizontal setup, the divider is flush with the middle of the front and back panes of the aquarium glass. A vertical gyre tank is good for keeping tall coral species such as gorgonians, arborescent soft corals and tall staghorn corals. This type of aquarium can be more aesthetic because it is easy to hide the pumps and flow outlets behind the divider of the aquarium and it preserves the viewing area of the tank. However, since the water mass of a vertical gyre is always in contact with the bottom of the aquarium and the surface of the water, it has more potential to develop velocity shear with faster flow at the top and slower flow at the bottom. In a horizontal gyre tank the powerheads or flow outlets are placed underneath the divider and they are aligned in a horizontal plane. This setup preserves the original actual surface area of the aquarium but it does so at the expense of the height of the aquarium. The larger surface area and closer proximity to the lighting source makes the horizontal gyre tank ideal for concentrated efforts of coral culture.
Figure 2 is an example of a horizontal gyre tank which I built for stony coral culture. The aquarium is 33 gallons, 4 feet long, 14 inches wide and 12 inches tall. The divider was made out of two pieces of dark plexiglass which were overlapped in the center. Both pieces of the divider were unattached and I found that I could vary the speed of the water flow by adjusting the distance of the gap between the divider and end faces of the aquarium glass. The water movement was provided by one Seio 820 pump on one side and two Maxi-jet 1200’s on the other side. A Chauvet light timer was used to alternate power between the pumps for 5 to 15 minutes to each side. Since the water flow was so unidirectional in this long aquarium, it was very simple to measure flow speed. Water velocity was calculated by adding neutrally buoyant particles to the water and timing how long it took for them to travel across a distance of the aquarium. Using this technique I was able to measure water flow speeds between 15-22cm/s throughout the entire aquarium. These velocities are within the range of ideal flow speeds for optimum particle capture, respiration and photosynthesis of many corals. Figure 3 is an image of a vertical gyre tank built and designed my Michael Janes of Aquatouch. Mr Janes is an octocoral specialist and he refers to his design as a laminar flow tank. He designed the aquarium to produce ideal flow conditions while still maintaining enough vertical space to accommodate tall soft coral species such as gorgonians. Although this aquarium was designed primarily as a proof of concept, Mr. Janes continues to work with this type of gyre tank for studying octocoral species.
Figure 3 A vertical gyre tank (a.k.a. laminar flow tank) designed to accommodate tall coral species. Photo by Michael Janes.
Gyres in Reef AquariumsAn aquarium does not necessarily need a divider to produce gyres of the water mass. Although the water movement will not be as complete and uniform as it is with a gyre tank, it is still advantageous to encourage water movement to follow a circuitous path. In a reef aquarium with live rock and coral on the bottom, the water surface of the aquarium provides the least resistance to moving water. Because of the lack of friction, moving water which is directed in this region will produce the most momentum of the water mass. If there is an even transport of the surface water from one side of the aquarium to the other, the entire water mass should begin to gain momentum as it is moved at both ends. At one end of the aquarium, the water will begin to “pile up†and then sink down. At the other end, water will rise up to replace the volume which is displaced by the water motion. Although it is easiest to create gyres which follow the top and bottom surfaces of the aquarium, this is not the only way to create gyres. Figure 4 is a photo of a 180 gallon aquarium with an overflow drain right in the center of the tank. This aquarium contained a modest amount of live rock and it was circulated by encouraging mass water movement around the center overflow. Once again a Chauvet light timer was used to alternate the flow between two circuits of powerheads. Each circuit contained pumps which were diagonal to each other and in this fashion the force of both pumps were working together to move the entire water mass. The center overflow was not necessarily the most aesthetic design but it was very easy to spin water around it using only very modest water pumps.
Figure 4 Because this 180 gallon was circulated using mass water movement techniques, only modest equipment was required to produce adequate water motion.
Not only can mass water movement techniques help aquarists produce higher water flow speeds in the aquarium but it can also encourage more water movement through the interstices of live rock and corals with open growth forms. Normally an aquarist might target one or more plumes of water movement at corals which require fast water flow speeds. In this scenario, the turbulent water flow plume encounters a lot of friction on its way to the desired location of the reef aquarium: it will experience resistance from the still water around it, it will experience drag from the shape of the corals it encounters and the turbulent nature of the water flow plume will do little to preserve the momentum of the water movement. In a scenario with the employment of mass water movement, the behavior of the fluid will be much different. The plume of water motion from the same source will encounter less resistance from the water around it since both parcels of water are moving in the same direction. The decreased resistance will straighten out the flow and preserve more momentum. Not only will the water be moving faster once it reaches a coral, since water is moving away from the coral on the downstream end, water will be forced through the normally stagnant water which is present at the interior of corals with open growth forms. Figure 5 shows an aquarium where dense coral growth account for a significant portion of the aquarium’s cross section. When using mass water movement techniques in cases of dense coral growth, water flow speed can actually accelerate as more water is pushed through spaces with a smaller area. Aquarists who wish to encourage additional water movement at the inside of dense coral colonies will see great benefits from using mass water movement techniques.
Figure 5 An example of a mature reef aquarium which exhibits very dense stony coral growth.
ConclusionsThe reef aquarium hobby has a long way to go before our understanding of water flow catches up with what we know about reef aquarium lighting. Like the “Watts per Gallon†moniker that came before it, the use of “turnover rate†to describe water movement continues to cripple the progress of more advanced water movement techniques. By encouraging the formation of one or more gyres, aquarists are capable of producing more water movement in terms of overall water flow speed. Since higher flow speeds produce greater amounts of turbulence, this translates into increased gas exchange and higher rates of photosynthesis and respiration.
