By Richard Harker PhD

Water motion is very important for sessile (immobile) animals such as corals. Water brings nutrients to the animals and carries away waste products. The challenge in reef keeping is creating realistic water motion in a reef tank with the tools commonly available to the hobby. (For a more extensive discussion of water motion on the reef, see my earlier Aquarium Frontiers article on water motion in the archive section.) Most hobbyists use submersible pumps such as powerheads placed around the perimeter of the tank to create water motion. The problem with powerheads is the narrow stream of water these pumps produce. Like the water leaving a garden hose, water leaving the nozzle of a powerhead is unidirectional. This type of flow creates a few isolated areas with intense flow, potentially causing harm to delicate coral tissue. In contrast, water motion on the natural reef is much stronger, but diffused.

Figure 1

Companies that serve the reef hobby have created various tools and devices in an effort to overcome the limitations of using pumps for water motion in a reef tank. The simplest adaptation is the wave maker or wave timer. Devices like Red Seas WaveMaster and Aquarium Systems’ Natural Wave Timer periodically switch pumps off and on to better simulate random water motion in a reef tank. (A future column will look at these types of devices to determine whether they are necessary.) A more elaborate solution offered by several companies is to vary the direction of the flow of the water much like a rotating lawn sprinkler. One approach is to rotate the nozzle of a powerhead using a motor. While the approach works, reliability has been a challenge. The failure rate of the pumps has discouraged many hobbyists from trying them. A second approach has been to use the water flow of the pump itself to power the movement of the pump.

Ocean Currents (www.ocean-currents.com) manufactures three wave and current generators that take this second approach. Water flowing through the water generator powers a rotating device that varies the direction of water flow. One generator is the Rotator. The flow of water quickly spins a half inch elbow sitting at one end of the generator. A second unit, the Oscillator, looks similar to the Rotator but uses a ratcheting mechanism to slowly rotate the half inch elbow. A third device, the Pulser, has a slot on the side of the generator rather than an elbow. Water flows from the Pulser in a broad stream. All three have a threaded one half inch intake, and include a barb adapter. Four small suction cups on the bottom (or top as one would normally mount a powerhead) are provided for mounting. Accessory brackets increase the options in mounting the unit, although they do add to the cost of the unit.

Figure 2

While the devices differ in their operation, installation is similar for all three. Ocean Currents recommends a minimum flow rate of 400 gallons per hour, and that the water flowing to the unit be prefiltered to reduce the need for cleaning. A great deal of effort has gone into the design and construction of the units. All three generators are solidly built using heavy duty materials. Each comes with a 30-day limited warranty, and repairs after the warranty has expired are a flat $29.95.

To evaluate the three generators, a 50 gallon test tank was set up with an Eheim 1060 submersible pump feeding the generator through a short section of Eheim tubing. A bracket mounted to the side of the tank enabled me to install the generator 2 inches below the surface of the water. Water motion was measured in two ways. To gather quantitative data on water flow, several test cubes were placed throughout the bottom of the tank. The dissolution of test cubes has been used extensively to study turbulent water motion (see the references in my article mentioned above for more information). To gather qualitative information about water flow throughout the tank, weighted test strips using narrow strips of buoyant packing material (bubble-wrap) were placed throughout the tank. This enabled me to detect water flow direction at all depths.

Figure 3

All three devices performed as advertised. The Rotator rotated, the Oscillator oscillated and the Pulser pulsed. In appearance, all three functioned as they should. The data, however, painted a different picture. In both the quantitative test as well as the qualitative test, the pumps created very little water motion. The water energy generated was no greater than one would find with a small powerhead mounted in the corner of the tank. At first, I thought the Eheim pump was at fault and replaced it with another pump. The outcome was the same. Only a fraction of the pump’s rated water flow reached the output.

Figure 1 is a captured frame from the videotape of the Rotator in action. The test strips are upright showing little deflection from the water flow. Contrast the lack of deflection in Figure 1 to the greater deflection in Figure 2 where water flow is generated using two small powerheads continuously running.

Figure 3 is the energy distribution of the Rotator. The X and Y axes (horizontal dimensions) of the chart represent the tank. The Z axis (vertical dimension) represents the water energy using a dimensionless scale based on dissolution of the test cubes. The water energy is evenly and broadly distributed, so the Rotator does effectively broaden the flow over a conventional pump, but it does so at the cost of significantly reduced flow. Figure 4 is the resultant water flow when the Eheim pump is simply mounted on the side of the tank. Water flow is more focused, with the highest energy on the other side of the tank as the stream of water strikes the side of the tank and travels down the side. However, water energy in even the least turbulent areas of the tank is substantially higher.

Figure 4

The Eheim 1060 is rated at 600 gallons per hour, so it meets the stated requirements for any of the generators. The pump, however, is only rated for 10 feet of head pressure. In other words, the pump can only lift water to a height of 10 feet. This appears to be the problem. The design of the units creates significant resistance. Water flow provides the energy to rotate the return, The resistance of the rotating PVC, combined with the resistance of the water in the tank against the rotating elbow, creates a great deal of back pressure. Compensating for the high flow resistance requires a pump with higher maximum head pressure than the Eheim can provide. To create sufficient water flow in the average size tank, a pump with a high maximum head is required. For example, the Little Giant 3MD-SC has a rated capacity of 665 gallons per hour at 6 feet. Unfortunately, Ocean Currents discourages the use of a pump that generates more than 5 psi, eliminating the 3MD-SC from consideration because it can generate 9.5 psi. If one steps down to the Little Giant 2MD-SC, the maximum pressure of the pump drops down to a more reasonable 6.3 psi, but rated water flow drops to 465 gallons per hour at 6 feet. Time limitations did not allow me to try this combination, but based on my observations, this would seem to be a reasonable combination.

The Ocean Current Wave Generators should work as promised if used with the properly sized pump. The key question is whether the performance of the Ocean Current wave generators justifies the investment necessary. Very few submersible pumps are designed to pump against significant back pressure. Only the largest and most expensive will generate enough flow. A better match with the Wave Generators is a pressure-rated external pump, and using one requires additional plumping. Given the plumbing and pump requirements, maximizing the benefits of one of the Ocean Current units could get expensive. For the average size tank, at least two units would be advisable, so one might use a larger external magnetic drive pump with the output feeding both generators. Even then, the amount of current generated in the tank will approach the level one would find using two small inexpensive powerheads running continuously.

It should be emphasized that the Ocean Current units are well thought out and solidly built. The limited flow is an unfortunate consequence of using water to power the unit. Future columns will look at other design approaches that integrate a powerhead into the generator itself.