August 14 2007,


Anyone who has been scuba diving or snorkelling will tell you that despite its calm appearance, the water near a reef is constantly in motion. This is especially true in the fore reef, where a lot of SPS corals come from. Surprisingly, when it comes to setting up a reef tank, water movement is often overlooked. This is unfortunate especially since creating adequate water movement can be one of the least expensive aspects in reefkeeping – and one of the most important. It is no exaggeration to say that proper water movement goes a long way towards helping a reef system thrive.

By: Mike Paletta

Keep it moving…
Proper water movement is important for a number of reasons. First, organisms on the reef are accustomed to an environment with such movement. As a result, they have developed physiologically to make use of the water moving around them.

This is especially true of the sessile invertebrates such as corals and clams. They use the currents which bring food, oxygen and nutrients to them and carry away their waste products. This is why most corals grow perpendicular to the current.

Several authors have reported that new growth of captive Acropora colonies are often spindly and slower when compared to that in wild colonies.

Many hobbyists have experienced this with low water velocity flowing across stony coral colonies.

Yet once the flow is increased, their original growth pattern and the speed of growth may increase. This flow not only increases the thickness of the new growth, but the previously spindly growth may thicken as well.

An example of the need for strong water movement is shown with Acropora palifera.

This species normally grows as thick, unbranched colonies on reefs where wave action is greatest. But when placed in tanks with a reduced water flow, they grow in a more branched manner, similar to other Acropora species.

Xenia colonies also grow faster when the current stimulates them. When water movement is low, the colony has short polyps close to the stalk.

Once the water movement increases, the polyps extend from the main body to the point that they are four times further out.

Conversely, when the current diminishes for these long-polyped colonies, and even though light and water quality remain constant, the polyp length again shortens.

This shorter-polyped colony also reproduces at a much slower rate. I suspect this is because the long polyps can attach themselves some distance from the stalk before breaking off.

That polyp then grows into a new colony. The shorter polyps don’t have the same opportunity to attach far from the mother stalk. Instead, they rely on the stalk splitting in half to produce new colonies, which takes longer.

Some corals will also not grow, or grow more slowly, if adequate water movement is not provided. This may be because they did not receive sufficient nutrition.

More likely is that growth was stunted because waste products were not adequately removed. Water movement is important for cleaning as most corals have little capacity for removing waste material from their surface.

This is especially true of the small polyped stony corals. Because of their small amount of living tissue, they do not waste their energy removing waste material but depend on water movement to clean them.

In a study at Eilat in the Red Sea, it was found that soft coral colonies located in areas of strong water movement (and therefore with low sedimentation) grew three times faster than did colonies where sedimentation was a problem.

If such a build-up occurs on stony corals, they will bleach. In soft corals, the affected areas will rot or blacken and eventually die.

Areas where this detritus builds up also become prime breeding grounds for algal blooms. Good water movement within a reef tank will keep this detritus in suspension long enough so that much of it can be removed by the filtration system or so that it settles in the sump where it can be removed later.

Good water movement can help eliminate or reduce algae in areas where it has become established. In one instance, a colony of green star polyps was covered by a thick mat of hair algae, which was slowly killing the colony.

After removing as much algae as I could, I placed it in another tank next to a similar colony that was thriving. Shortly after, the polyps opened and within two weeks, the algae was no longer a problem.

The only difference between the tanks was that the star polyps were exposed to a vigorous current and lowered nutrient levels.

Another overlooked aspect of water movement is its effect on a fish’s health. If you look at the labels of many fish foods, you will find a large percentage is fat.

Now fat is important in nature as it is quickly converted into energy – a bonus as fish in the wild are for the most part very active and require large amounts of energy. If fish are placed in an environment where they don’t have to battle water currents, fat could build up in the body with disastrous results.

Bubble, bubble…
There are three main types of water movement – laminar, surge and turbulence flows.

Laminar flow is a straight flow, like that produced from a powerhead or at the end stages of a wave whose energy has been channelled in one direction by the reef.

To an observer looking at a school of fish, surge is when the shoal remains in the same pocket of water, but suddenly appears to move 1.8m/6′ in one direction and then just as quickly back again.

Turbulence is the random flow of water in multiple directions. Of the three, turbulence is the most desirable in a reef tank – and the most difficult to produce.

More power
A powerhead is usually the easiest way of introducing movement. They are relatively inexpensive and can be controlled to produce varying amounts of water movement.

Their main shortcoming is that they only produce laminar currents, and these can’t be aimed directly at an invertebrate as the force is so great that the coral will either close its polyps, or in the worst case may shear the tissue right off.

However, with a little ingenuity and some additional electronics, a powerhead can be modified…

On most powerheads, it is possible to attach PVC fitting to the nozzle where water is ejected. This will help diffuse the energy on the outflow. In order to get the PVC fitting to fit snugly, it may be necessary to sand or file the inside of it for a precise match.

It is then possible to spread the flow out from this nozzle by using T or Y fittings and reducing the bushings to distribute the flow. Doing this produces a more gentle flow through several nozzles that won’t cause the coral to close up.

The downside is that doing this reduces the flow rate and over time may shorten the life expectancy of the pump. As the current is laminar, it may also affect the direction in which some corals grow.

However, the plus side is that you can direct the current at a specific spot to reduce detritus build-up, or direct flow behind rocks to prevent detritus from accumulating.

A new device has recently come onto the market which allows this flow to be directed over a wider area. The SeaSwirl oscillates 180 degrees so that the flow covers a much broader area. It can be hooked up to either a powerhead or the return pump.

When several of these devices are hooked up, flow patterns can be set up so that they produce turbulence similar to that found on a reef.

Electronic wavemakers
To add to this turbulence, the powerheads and SeaSwirls can be coupled with an electronic wavemaker that shuts the powerheads off and on in random fashion. There are several wavemakers on the market, each with different attributes.

In general, the wavemaker is designed to switch powerheads on and off to mimic surge. In the better wavemakers, this is a gradual ramping up and down rather than simply turning off and on.

This â€˜soft start’ is advantageous in that it reduces wear on the pump. It also mimics what occurs on the reef. However, some powerheads are not designed to work with these types of electronics, so check with the powerhead manufacturer before putting a powerhead/wavemaker combination together.

Using a powerhead, SeaSwirl and a wavemaker may make it possible to not only produce surge but also turbulence. And better wavemakers can activate the powerheads randomly to simulate turbulence.

All that is necessary is to direct the flow from the powerheads either onto each other or so that at least part of the flow from one is in the path of the other.

There will still be laminar currents and surge as the powerheads come on, but as the different flows collide from multiple powerheads, a more random pattern of movement closely resembling turbulence should occur.

Cheaper methods
For those of you desiring this effect without the costly purchase of a wavemaker, there are alternatives.

A simple way is to direct the flow of the powerheads against that of your return pump and to program the powerheads on timers. Timers that work on short time intervals – five minutes or less – work best, but even those with 30 minutes or more can be used. The result is not quite as dramatic, but you will produce some turbulence.

The return flow from the pump can also be used in a number of ways to get good flow within the tank. If the goal is to get a surge from side to side like on the reef, this is possible. To do this, the pump flow needs to be split at a tee. After this tee, electronic ball valves can be inserted at either end of the tee.

These electronic ball valves can be programmed (I suggest a microprocessor timer) so that the flow passing through them will alternate from one valve to the next. The microprocessor allows you to adjust the time between surge changes.

To achieve the effect, the outflows should be directed opposite each other across the length of the tank. But this method is really for those with an unlimited budget – each electronic ball valve costs Â£119 or more, and the microprocessor is another Â£30 or so.

Pump devices
There are also several new pump-related devices that produce reef-like movement in the tank. In the first of these, a small pump or powerhead is placed within an acrylic box. Rather than having the flow come out via a nozzle, a device similar to a boat propeller is attached to the pump.

As the propeller rotates, it produces flow from a large area rather than a jet. This mimics what occurs when a wave comes across the reef.

These devices are quite impressive in terms of the amount of water they push. This movement is not as strong as that coming out of a small jet, but it is strong enough to remove waste from corals and to keep detritus in suspension.

When used with a wavemaker, several of these can produce patterns of turbulence on a smaller scale close in nature to those seen on the reef.

The eductor
An alternative is the eductor. It looks like a water-based venturi device and compresses water from a pump. When it expands from the venturi, it draws in more water with it so that the water coming out of it is expanding and not like the laminar flow that comes out of most nozzles.

These devices are attached to the outflow from a pump and create a wall of water. They have no moving parts and are relatively inexpensive to buy and use.

Their only short-coming is that they are rather large (15cm/6â€) and may be distracting in smaller tanks.

Carlson Surge Device
Then there’s the tried and true method of Dr Bruce Carlson, of the Waikiki Aquarium. It’s gained such a following that it’s now called the Carlson Surge Device.

A large, tall reservoir (the surge tank) is placed above the aquarium. A large diameter piece of PVC pipe runs from the reservoir into the aquarium, positioned about 5cm/2â€ under the water surface. Using two 45-degree fittings on this pipe enables it to bend neatly to the aquarium.

A second, smaller-diameter tube runs from the aquarium to the reservoir. The system works by the use of gravity and the creation of a syphon.

Water from the tank is pumped via the smaller tubing into the reservoir via a powerhead. When the reservoir is filled with water, the larger pipe fills with water and a syphon action empties the reservoir into the aquarium until the former is empty.

When this happens, the pipe sucks in air and stops. All the while the powerhead continues to draw water until the reservoir is full again and the whole cycle repeats itself.

The flow depends on the size of the reservoir, its height above the tank and the diameter of the pipe extending into the reservoir.

Armed with the knowledge, it should now be easier for you to decide on a method to create water movement in your reeftank – you will be pleasantly surprised by the difference it will make.

Since this article was published several more gadgets have been released for improving flow rates in reef aquaria. Check out the Tunze Turbelle Stream pumps, the smaller Tunze flow pumps, Hydor Korallia pumps, and the impressive Tunze Wavebox.


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