Credit: Robert Michelson
Depending upon what you are trying to propagate in your system, water current can be of varying significance. For fish-only tanks, intra-tank currents may be of little consequence. If your tank is simulating a tide pool or lagoon, very low or sporadic currents might be suitable. As you desire to propagate creatures that normally live in tidal zones or reef flats bounded
by a barrier reef, you will want to seriously consider additional means of current generation beyond that constant cross-flow created by your circulation pump. Of course if you are into the small-polyped stony (SPS) corals that frequent the barrier reefs of coral atolls, you will have to go to extraordinary measures to move a large volume of water in a random fashion to simulate the surging currents found on the reef.
Why do some sessile creatures need current? Often they require flowing water to not only bring them their food source, but more importantly to wash away the concentrated waste products exuded from their bodies as a result of their normal metabolic processes. Some creatures will literally drown in their own waste products if they are not washed way by some outside influence (ocean currents).
Certain species, especially Xenia, are thought to facilitate the waste elimination process by rhythmically pulsing the extremities of the polyp. Even so, pulsating Xenia still prefer significant current in order to thrive. Other species of soft corals expand and contract their entire bodies on a diurnal cycle which this author believes to be an aid in shedding waste products from their outer layers while also dislodging entangled detritus, silt accumulations, and perhaps ectophoretic creatures which have sought refuge amid feeding tentacles during the day. The Sarcophyton (leather) coral is an example of a sessile creature that can modify its texture to become very smooth and contracted to allow currents to cleanse its surface.
Ocean currents originate from several sources. Lunar- and Solar-induced tides are generators of sustained constant (but regularly alternating) currents. Wind-driven tides, known as ‘seiches’, create a similar effect. Estuarine currents are caused by runoff of fresh water rivers as they enter the coastal oceans over the planetÕs continental shelves.
Lesser known sources of current generation to most aquarists are the various density currents resulting from temperature and salinity gradients in the ocean. As polar ice forms from sea water, 70 percent of the sea salt is expelled. As the ice thaws and refreezes, it becomes more pure, eliminating 70 percent of its remaining salt content on each refreezing cycle. The eliminated salt goes into the unfrozen sea water making it more dense, so it sinks. The sinking higher density (saltier) sea water creates a current as it sinks and flows along the bottom away from the polar regions toward the equator. Most of this current activity occurs at great depths over the abyssal plains and does not affect the kind of creatures that we normally keep in our aquaria, but salinity gradients do exist closer to land.
For example, the Potomac River which flows from the Washington D.C. area out into the Chesapeake Bay and ultimately the Atlantic Ocean, has a salt water wedge that is a density gradient countercurrent which flows up stream along the bottom of the fresh water Potomac River. This salt wedge extends all the way up into the tidal basin around which the cherry blossoms of our nationÕs Capitol bloom.
Similarly, density gradients can be caused by temperature differences in ocean layers. These can be local, such as the night time cold currents which come in after dark to bathe the coelenterates along Florida’s John Pennekamp Coral Reef. Globally, some of the strongest currents in the world result from density differences aided by the wind (e.g., the Gulf Stream and the Kuroshio current).
The Sun is the energy source for most of the world’s currents (except for gravitational tides or anomalous phenomena like tsunamis). The Earth is a great heat engine driven by the Sun’s radiated energy. This creates violent storms in the Earth’s atmosphere which in turn impart energy into the ocean. Most wave action is wind-driven. Storms in Antarctica create waves that propagate all the way to the tropics. Wind-driven waves are the major source of surge over the tropical reef systems of the world. Wind-driven wave surge causes a circular motion of water molecules in the vertical plane. When this wave action encounters an obstruction such as a reef, a back and forth surging of the water is manifested over the reef. As the water column (surface-to-bottom) becomes too shallow to support waves of a particular wavelength, the wave will rise up and ‘break’ across the obstruction in the familiar fashion sought by surfers. This is true for large, long period waves. The tiny waves on the surface of the ocean (‘chop’) are generated locally by wind, and these too cause currents when passing over objects very close to the surface. The region in which surface wave action is instrumental in surge formation is called the mixing zone and extends no deeper than most photosynthetic corals live. Steady currents found at greater depths usually have their origins somewhere other than the local mixing zone.
Therefore, when designing a current generator for aquaria, one must consider in which region of the ocean the coral thrives, and what type of wave action is found there. Typically all of the coral animals found in tropical aquaria will come from the mixing zones of the world’s oceans and usually in the upper 10 meters. Wave makers for invertebrates coming from 30 meters of water will need zero to constant currents, while those coming from 10 meter habitats will prefer surging currents, and those living within meters of the surface may require large, random, multidirectional blasts of current to simulate violent surge and breaking waves.
Constant currents can be generated by a simple circulation pump, while surging currents are often implemented by cycling a pair of opposing circulation pumps to first pump water in from one side, and then the other. Oscillating power heads, depending on their placement, can also perform this function. Violent chaotic currents are frequently created with ‘dump buckets’ which slowly fill up and then discharge their contents into the aquarium rapidly by dumping or by valving through large diameter pipes.
Alternating pumps are expensive because two pumps are used to move a volume of water equivalent to what a single pump would move (since both pumps never operate simultaneously). A timing controller is also required to cycle the pumps. Oscillating power heads (or any power heads, for that matter) are ugly additions sticking down into your aquatic garden. Dump buckets are very effective, but require lots of space (usually above the aquarium) and may require additional pumps and valving. Worst of all, they are noticeably noisy!
My aquarium was designed to coexist with people, being placed in a wall between the family room and breakfast room. Though there was room for a modest dump bucket system, the noise and potential for splashing and salt creep made such a current generator ill advised. When designing the system I chose to replicate current conditions found deeper on the reef at a depth of about five meters.
At five meters one would expect some energetic surging action, but mostly a slow, relatively constant back and forth current with some static flow areas. This environment was achieved using one Little Giant 4MDQ-SC pump (about 640 GPH with a 12-foot head) and a Mag Drive 7 (about 700 GPH with no head). I designed a special valve that allows the single Little Giant 4MDQ-SC to supply a bidirectional sinusoidal flow alternately emanating from one end of the show tank to the other.
The special valve is driven by a small gearmotor taken from a surplus variac (motor-driven variable autotransformer) used in Atlanta’s low power TV Channel 57 transmitter. This motor was free, but almost any synchronous 60 Hz AC clock motor would have worked. The motor turns a geared output shaft at 1 RPM. This in turn drives a valve made from PVC pipe and a machined section of Delrin bar stock with orthogonal holes. As the Delrin cylinder rotates within the PVC structure, water from the single pump is allowed to flow from the single input port to one of the two output ports.
This results in full flow out each of the output ports once every 15 seconds. The flow intensity from each of the valve output ports is sinusoidal and phased by 90¡ relative to the 1 RPM rotational period.
Each output port is connected to one of the inflow pipes located at the ends of the show tank. When port one is open (and port two is closed), water flows from right to left across the show tank reef. When port two is open (and port one is correspondingly closed), water flows from left to right across the show tank reef. During intermediate states, both valve ports are supplying a portion of the maximum current flow that varies by the sine or the Delrin cylinder angle for port one or the cosine of the angle for port two. The valve is designed to deliver a flow corresponding to the relation below:
This means that the show tank receives the maximum possible flow at all timesÐ only the flow vector is changing.
This system provides a realistic wave action at less cost than alternately-switched twin pumps. A twin pump arrangement provides a binary flow which starts up abruptly and cuts off abruptly. Besides being harder on the pumps than continuous operation, the wave action produced is unnatural. Recall that natural wave action produces a rolling motion in the vertical plane. When a wave encounters the reef, it is transformed into a sinusoidal back and forth surge just like that produced with my valve design and a single pump. Twin pumps produced sudden blasts of full-force current that terminate just as abruptly.
Alternately-Switched Pump Action
Does this matter to the coral? Who knows… but the single continuously-operating pump/valve system is more economical and doesnÕt present the potential for electronic interference with radio and TV reception that exists when inductive devices such as induction motors (used universally to drive all brands of large aquarium circulation pumps) are repeatedly switched on and off.
Now let us consider the method used to cause short term high velocity turbulence which might be encountered at a depth of 5 meters as a result of nearby breaking wave action. Dump bucket noise was unacceptable and the use of power heads was aesthetically displeasing. My solution was to mount a circulation pump outside the aquarium with a clear suction pipe placed in one corner and a clear acrylic flow-delivery pipe leading to the geometric center of the top of the aquarium.
Central Distribution System (top view)
An acrylic distribution head connects to the flow delivery pipe through a freely rotating interface. The rotating interface is constructed from a PVC separable pipe union. The pipe union is threaded on both ends and has its stationary half fixed to, and penetrating through the 0.5 inch acrylic top of the show tank. The acrylic flow delivery pipe screws into the pipe union to form a watertight (but removable) joint.
Central Distribution System (view from water surface inside show tank)
The acrylic distribution head threads into the freely rotating lower half of the pipe union on the lower surface of the show tank top. Three exit ports direct water out of the distribution head. Two are angled down and away from the center of rotation to form an intense sweeping current across the reef surface as the distribution head rotates. The third exit port provides a tangential jet of water that drives the rotation of the distribution head at an approximate rate of 12 RPM while stirring the surface waters and causing turbulence on the upper sections of my reef.
Acrylic Distribution Head
The acrylic distribution head is invisible above the water’s surface and only the three clear acrylic exit ports extend below the surface, and then only about one inch. The entire arrangement casts no noticeable shadow and provides periodic random high intensity currents in a most unobtrusive manner.
Presently this central flow system is energized under computer control for a few minutes each hour, with longer ‘on’ times during the mid day hours than during the night. The reasoning is that the ocean wave action is more energetic during the day time when the Sun creates thermals and gyres that churn the sea surface more actively than at night. Many of my corals are also retracted at night, so the periodic waste-removing cleansing action produced by this semi random current would have little effect after dark.
Aside from the very standard pumps employed, all of the current distribution systems described were homemade using surplus materials, pipe stock and fittings available from local building supply houses, and a measure of common sense.
Addition of Surge Devices
The rotating central circulation system worked well, stirring the water between the reef escarpments, but due to the rather high minimum rotational frequency of the central nozzles (approximately 0.5 Hz at its slowest), the current pulses striking the surface of the reef were unlike those occurring in nature. A longer period between current pulses seemed more desirable. In an attempt to simulate this, the pump driving the rotating central circulation system was timed to go on and off for periods of varying duration once per hour. This proved to be too stressful for the Supreme Mag-Drive 700 gph pump’s impeller. It failed after three months of intermittent use. A post mortem on the pump revealed that stress fractures occurred in the impeller blades due to the sudden shock each time the pump was energized. Once one blade broke away from the impeller shaft, it would interfere with the remaining blades and cause them to shear off as well. Based on the possibility that the original impeller was defective, a second impeller was installed under the pump warranty. Over a similar period of about three months of intermittent use, it too failed in the same manner.
Clearly the design of the Supreme Mag-Drive pump was not suitable for intermittent duty and something else had to be tried. It was at this juncture that I decided to try Carlson surge devices. These would have to be custom made to fit above the show tank in the limited space available on either side of the light hood.
A Carlson surge device provides an intermittent flow of moderate duration (a function of the surge device reservoir volume) and adjustable surge frequency (a function of the reservoir fill rate). The surge produced is fairly realistic in that it creates a constant mass flow during the surge, and can be timed to provide surges of duration similar to those encountered on the wild reef.
The surge device has no moving parts and does not require cycling of a pump. The surge device relies on the creation of a siphon to automatically drain a reservoir at a flow rate several times greater than its constant fill rate. As the pump draws water from the show tank (or other source within the closed circulation system), it is pumped into a reservoir that is physically above the show tank water line. A siphon tube extends from the bottom inside of the reservoir, up through the side of the reservoir (at a point about 95 percent up the height of the reservoir), down the outside of the reservoir, and into the show tank. The external end of the siphon must extend down into the water of the show tank in order to provide a slight back pressure which helps the siphon to start automatically. When the reservoir fills to the top of the siphon, the inner half of the siphon tube will also be full. As the pump continues to raise the water level in the reservoir above the upper bend in the siphon tube, water will begin to spill over the bend in the tube and flow down into the show tank. This will create a siphon action that rapidly drains the reservoir into the show tank.
Several parameters are critical to the correct operation of the surge device. First, the siphon tube must be of sufficient diameter to allow the reservoir to drain faster than it can be filled by the pump– otherwise the flow would simply be continuous and equivalent to that of the pump, or on the other hand, if the pumped flow is greater than the siphon flow, the reservoir will overflow.
The siphon tube must extend to the bottom of the inside of the reservoir otherwise the reservoir will never completely drain. Also, the siphon output must extend below the show tank water surface for the siphon to start automatically. Finally, as mentioned above, the reservoir must be physically higher than the show tank water level for the siphon to work.
Adjusting the flow rate into the reservoir is easily accomplished with a valve to restrict the flow. In the event that the siphon were to become clogged, the reservoir would overflow, so the reservoir should be a closed container that is airtight except for a vent hole at the top. Were the reservoir to overflow, the water would go out the vent at the top where it can be channeled harmlessly back into the aquarium by means of an overflow tube. During normal operation, this vent and overflow tube allow air to escape from the closed reservoir chamber as air is displaced by the incoming water. The vent also prevents a vacuum from occurring during the rapid outflow when the siphon is active. The overflow tube must be placed just above the surface of the show tank water and not allowed to extend beneath the surface. Otherwise, as the reservoir fills, air expelled through the vent would bubble out the end of the overflow tube. This would be noisy and not very attractive. Also during the siphoning of the water in the reservoir, water would either be sucked back into the reservoir through the overflow vent, or more likely, the operation of the siphon would be inhibited.
A point of major concern in the design of the surge device was noise mitigation. By making the reservoir totally airtight except for the pump input, siphon output, and the overflow vent, filling and siphoning noises are contained. The bottom of the siphon tube inside the reservoir is cut at a 2 degree angle relative to the flat reservoir bottom to allow the siphon to ‘break’ quickly without excessive gurgling. The pump fills the reservoir from the bottom to prevent the splashing that would occur were the water to fall in from the top. Finally, noise is abated by placing the reservoir on a foam pad to isolate internal sounds from being transmitted through the base into its supporting structure. As a result, these surge devices are silent.
In my 200 gallon reef system, two three gallon surge devices are used. To save money, a single 500 GPH 2MD-SC Little Giant pump runs both devices. One surge device is dedicated to each end of the show tank. Water volume pumped into each of the two reservoirs is adjusted by independent valves. The siphon tubes are 1-inch schedule 40 PVC pipe which is split just above the water’s surface and distributed through equal 3/4-inch pipes to either side of each end of the show tank. Loc Line segments terminating in a 3-inch fan diffuser allow the output to be directed on the sides of the reef at either end. Only the edges of the black Loc Line fan diffusers penetrate the surface of the show tank, the rest of the distribution tubing just above the water surface is invisible to those viewing the show tank. The input to the pump is a clear acrylic tube of 1-inch diameter that extends to a depth of about nine inches in one corner of the show tank. A clear plastic grate prevents the inhabitants from being sucked into the continuously-running pump. To prevent any pump noise, it is mounted on a concrete wall adjacent to the aquarium command center in the basement.
The reservoirs are rectangular boxes having all joints and ports secured with Weld*on-40 (‘liquid Plexiglas’). Because the reservoirs are tall (approximately 24 inches), internal cross buttresses of acrylic have been included during the construction to keep the reservoirs from fatiguing after repetitive filling cycles. To further assure that the thin wall (0.125-inch) acrylic does not flex, aluminum angles were cemented to the outside corners using the flexible silicon adhesive marketed in the U.S.A. under the name ‘Goop’.
The surge devices are adjusted to trigger every 30 to 45 seconds with a surge lasting approximately 30 seconds. They trigger asynchronously and provide a realistic periodic surge over the upper and inner sides of the reef escarpments. This has greatly improved show tank circulation and augments the more general sinusoidal back and forth surge from the primary circulation pump and its alternating flow valve. That circulation is improved is without question, as green star polyps which used to languish, now spread along the rocks in the direction of the flow.
There are some things to be considered when using the surge devices. First, my system was sized with adequate sump volume to accommodate back-flow from the upstairs distribution pipes in the event of a power failure. With the inclusion of the two 3-gallon surge devices, as much as six gallons can be suspended above the show tank at any time. During a power outage, these reservoirs drain back through the surge device pump into the show tank. The show tank can not accommodate the extra six gallons, so it goes over the overflow and down into the sump. Since the sump was originally sized to accept only the back flow from the distribution system pipes, the addition of six extra gallons of water is too much and the sumps overflow.
In anticipation of this an overflow pipe has been added to allow the extra water in the sumps to be diverted into a reclamation drum. When the power is restored, the entire system functions correctly, but the sumps are six gallons lower in level than they should be. A float switch in the reclamation drum then activates a pump that returns the six gallons back to the system. Were this to fail, the automatic refill system would gradually replace the six gallons of salt water with six gallons of deionized fresh water, reducing the overall specific gravity of the system by 3 percent. In a new system, this could be avoided by using deeper sumps.
Another consideration is that as the reservoirs fill, the amount of water leaving the show tank is diminished. As a result, the level of the sumps will drop. When the surge devices trigger, more water will be delivered to the sumps causing the level to rise. In the worst case, both reservoirs fill simultaneously and trigger simultaneously. This causes about a 0.25-inch change in the show tank water level and about a 1-inch change in the sump level. Since this occurs over a period of about one minute, the automatic water replenishment system is not activated because the sensory time constant for that device is much slower. Were one surge device reservoir to fill while the other triggered, there would be no net change in water levels. Since the operation of the two surge devices is asynchronous, changes in show tank and sump levels are rarely noticeable.
A final note is that at the beginning of a surge cycle, and again as the siphon ‘breaks’ at the end of a surge cycle, tiny bubbles are entrained in the surge flow. These typically last for only a moment, but some find them objectionable. In reality real wave surges often do entrain air in the form of tiny bubbles that are driven beneath the surface when a wave encounters a shallow reef.