Last month we looked at Ocean Current Wave Generators as a tool to create water motion in a reef tank. The Ocean Current devices use water flow to power a rotating nozzle that varies the direction of water return. As I noted in that review, the use of flow to power the movement creates a great deal of back pressure that significantly reduces the flow from the pump. This month we look at two water motion devices that use electricity to power a moving return.

The Sea-Swirl Rotating Return Device (www.sea-swirl.com) is similar to the Ocean Current Oscillator. Water from a pump is fed into the top of the unit by way of a hose barb, and the water exits through a fitting at the bottom of the unit. The fitting is an elbow that directs the flow horizontally across the top of the tank. Flow is determined by the size of the pump connected to the unit. The significant difference between the Ocean Current unit and the Sea-Swirl is how the rotator is powered. While the Ocean Current Oscillator, like a lawn sprinkler, uses the power of the water flow to rotate the return, the Sea-Swirl uses an electric motor to power the rotating return. The greatest potential advantage of doing this is decreased back pressure and increased flow. Water flowing through the rotator isn’t doing as much “work” and therefore more water motion energy (flow) enters the tank.

The Ocean Current units all have ½ inch fittings and are therefore limited in the amount of flow they can generate. The Sea-Swirl comes in three sizes accommodating ½ and ¾ as well as 1 inch fittings. Recommended pump sizes range from 550 gallons per hour (gph) for the #189; inch unit to 1150 gph for the 1 inch unit. Consequently, the Sea-Swirl offers the hobbyist more flexibility in that respect. One limitation of the Sea-Swirl is placement. While the Ocean Current units can be immersed and therefore be placed inside the tank, The Sea-Swirl rotating unit is not water-proof. It must sit above the tank mounted on the side with four plastic set-screws. Hobbyists with close fitting lighting canopies over the tank may find mounting the Sea-Swirl a little bit of a challenge. Mounted on my test tank, the return sits just below the water surface.

A return close to the surface gives the illusion of great water motion, but in reality this is not the best means to return water to the tank. Flow lower in the tank is more useful than flow at the surface, so a deeper return would have been better. One could fabricate some sort of deflector to lower the return level, but the manufacturer discourages this. The Sea-Swirl rotates 90 degrees, but the company also sells a Y connector to effectively create 180 degree coverage.

The construction of the Sea-Swirl appears typical of hobbyist oriented equipment. The case is made of lightweight plastic and the fittings are standard schedule 80 PVC. The elbow return is threaded and the unit includes a nozzle that threads onto the elbow to narrow the return. A narrow nozzle will increase the velocity of water flowing from the return, but will also increase back pressure and reduce flow. The only rationalization for increasing the velocity of the water would be if the velocity of the water was too low to reach to the other side of the tank. As I will discuss in a moment, this wasn’t a problem with the Sea-Swirl.

The other unit I evaluated was the Zoo-Med Power Sweep, a rotating powerhead. The Power Sweep combines a submersible powerhead with a rotating nozzle that rapidly sweeps about a 30 degree arc. The Power Sweep comes in four different sizes generating from 125 to 270 gph. The unit is typical of powerheads in its price range. It has an aerating option common in freshwater applications, a nozzle defector to spread the flow horizontally, and the option of attaching a prefilter to the intake of the pump. Perhaps the weakest aspect of the Power Sweep is the limited mounting options. The Power Sweep is mounted to the tank with four small suction cups. Suction cups are notoriously famous for failing over time, letting the pump fall or shift. The most attractive aspect of the Power Sweep is the unit’s price. It costs no more than a comparably sized powerhead, essentially throwing in the rotating return for free.

Methods for evaluating water flow over a coral reef have generally focused on measuring water velocity using a flow meter, or total water energy over time using dissolving test cubes. While both methods have proved useful for evaluating flow over a natural reef, they have presented problems for hobbyists attempting to compare water flow in their tanks to flow across true reefs. In the Ocean Current review I introduced a new technique of using buoyant material (bubblewrap) to monitor both the direction as well as force of water flow in a reef tank. Monitoring the direction and degree of deflection of the test strips provides a useful qualitative comparison between reef tank setups. Water flow in the reef tank is video taped, and periodicity and maximum deflection from vertical of the test strip are determined by analyzing captured frames.

While this method can provide useful comparisons between tanks, it is also necessary to have relevant natural reef data to make the comparisons more useful. To this end I digitally video taped test strips placed around natural reefs where large healthy stands of stony and soft corals were growing. This enables us to compare water flow in a reef tank to water flow on a natural reef in real time. I discovered that flow over large Acropora has three primary components. There is a gentle flow around the coral head that causes the test strip to swing back and forth a few degrees from vertical every two to three seconds. Approximately every 10 seconds there is a second much stronger surge that deflects the strip upwards of 30 to 40 degrees. This is the surge that divers note near coral reefs. The third component is tidal flow. Twice a day a unidirectional flow moves across the reef with such force that the test strip is deflected nearly 90 degrees from vertical.

This new real-time perspective of flow over a coral reef provides much more detailed information on the types of water motion we should re-create in a reef tank. Gentle water motion (less than 5 degrees deflection) should occur regularly, periodically punctuated with less frequent but more intense deflection (30 to 40 degrees) and even less frequent unidirectional high velocity flow.

To make the comparison between the Ocean Current Oscillator and the Sea-Swirl fair, a #189; inch Sea-Swirl was installed in the same 50 gallon test tank in which the Ocean Current units had been evaluated. The Sea-Swirl was fed by the same Eheim 1250 pump rated at 320 gph used in the earlier evaluation.

As readers of the Ocean Current Wave Generator review may recall, there was very little deflection of the test strips. Maximum deflection of less than 5 degrees occurred at the opposite end of the tank. Closer to the return there was hardly any deflection. This lack of flow close to the pump seems common in tanks using small powerheads at one side of the tank. Because the powerhead is typically mounted at the surface of the water, a wavefront flows across the surface of the tank strikes the opposite wall, travels down the wall and dissipates before it can make the return trip. Consequently, the weakest flow in this set up is the area directly below the pump.

A similar phenomenon was observed with the Sea-Swirl. The test strip in the opposite corner deflected up to 5 degrees, but adjacent strips barely moved and strips further away didn’t move at all. The slightly slower movement of the return seemed to generate more localized energy than the Ocean Current Oscillator, but the overall effect was very similar to the Ocean Current unit.

This was unexpected. My hypothesis was that a motorized rotator would be more efficient than a self-powered rotator, because less water energy would be dissipated in rotating the plastic parts. It now seems that the greatest resistance to flow is the two right angles the water must flow as it enters and exits the rotators. One 90 degree elbow is the equivalent of 4 feet of lift, and the combined resistance of two #189; inch elbows seems to significantly reduce flow of the Eheim or any similar pump. By comparison, the rotating mechanism apparently consumes much less energy.

The Power Sweep also proved to be a surprise. The 270 gph unit generated nearly as much water motion as the Eheim 1250 and Sea-Swirl. The fast moving rotating nozzle produced a different pattern of water motion in the tank, with more test strips deflecting but none deflecting as much as with the Sea-Swirl. As a single stand-alone unit, the Power Sweep would produce more uniform water motion than the Sea-Swirl, but less peak energy. The greater efficiency seems to be a result of the design of the unit. Water flowing from a powerhead flows directly from the impeller out the nozzle. It does not have to make any bends along the way as in the other units. There is less back pressure and a smaller pump can proportionately move more water. The limited sweep of the Power Sweep also aids in maintaining flow. Water is redirected no more than 45 degrees at the end of the sweep, and most of the time less than that.

After the Ocean Current review, a number of hobbyists wrote telling me of the difficulty they have had with rotating units. Long term survivability seems to be the greatest concern with any rotator. Organic build-up and the occasional unwary Astrea snail seem to take their toll on the devices. Each manufacturer cautions users about taking care in cleaning and maintaining the units. Their liberal inexpensive repair policies are further evidence that the makers of these products recognize that durability remains a challenge. On the other hand, I received a number of messages from happy users who have had great success with rotating units. Both the Sea Swirl and Power Sweep are warrantied for one year.

So which unit does the best job of moving water in a reef tank? That has proved to be a much more difficult question to answer than first thought. The Power Sweep is the most cost effective solution, offering varied flow for little more than the price of a powerhead. For tanks smaller than 55 gallons, a pair of the units would provide adequate water motion for a lightly stocked, predominantly soft coral tank. Placing two small additional stationary powerheads in a tank would be even better and still be less expensive than any of the other solutions. Using a combination of stationary and rotating pumps can provide the most cost effective solution for most medium to small reef tanks. Adding a small continuously running powerhead opposite the Sea-Swirl balanced the test tank circulation so that all areas of the tank received similar water energy.

In larger tanks, particularly those tanks predominantly stocked with small-polyped stony corals, exclusive use of the largest Sea Swirl rotators would probably be sufficient to create adequate water motion, but at significant cost. A pair of 1 inch Sea Swirls at the maximum rated flow of 1150 gph would require something like a pair of Little Giant 4-MDQX pumps, raising the total cost of water motion to close to $500. Using Ocean Current Oscillators to generate similar water energy would require the use of at least four and possibly as many as six units. This would generate the most random flow of any setup, but at a cost that seems out of line with the benefit. For systems larger than 250 gallons, the best solution is large powerheads like Gemini pumps or external circulation pumps. The pumps can be switched off and on with a simple appliance timer. For tanks longer than 6 feet, however, continuously running pumps are even more effective. These solutions generate as much flow for as little as one-quarter the cost of oscillators with external pumps.

Every tank’s needs are going to be somewhat different. The best solution is to construct a number of test strips and place them at various locations in the tank. Set up a pair of powerheads for water circulation and watch the effect on the test strips. Strive for at least 10 degrees of frequent deflection, and position the pumps so that the direction of the deflection regularly swings from one direction to another. Add a single rotating or switched pump to get the occasional 20 to 30 degree deflection. Add pumps as necessary to achieve sufficient movement. The simplest combination of pumps that can achieve this type of water circulation is the best solution for your tank.