Here is from article by Mike Paletta on reef flow......
Until recently the one aspect of a reef tank that was most often overlooked was water movement. Creating adequate water movement can be one of the least expensive aspects of setting up a reef tank, but it is one of the most important. Proper water movement is important for a number of reasons. First of all, the water around a reef is in constant motion; therefore, it is logical that the organisms present on the reef are accustomed to an environment with strong water 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, but even fish do better when they have current to swim in. The currents bring these organisms food, oxygen, and nutrients, and also carry away waste products. This is why stony corals do best in areas of the reef with strong water motion. Most algal blooms result due to excessive nutrients. In reef tanks these patches of algae usually are in spots where there is little to no water movement. As a result, detritus settles in these spots. That is why it is necessary to get good water movement over these areas. By doing so detritus remains in the water column where it can be filtered out. Water movement is considered adequate if the flow within a tank is between 5-10 times the tanks volume per hour. That is if a tank is 100 gallons in size the flow within it from the pump and powerheads should move at least 500 gallons per hour. The more often the water is turned over, the better the flow is considered.
There are three main types of water movement on the reef, each with very different characteristics. These flow patterns are: laminar flow, surge, and turbulence. Laminar flow is straight unidirectional flow like that produced from a powerhead, or at the latter stages of a wave whose energy has been channeled in one direction by the reef. Surge is similar but on a larger scale. Turbulence is the random flow of water in multiple directions. Of the three, turbulence is the most desirable and the most difficult to produce.
For most tanks, the powerhead is the most readily available means for introducing water movement into a tank. Powerheads are relatively inexpensive and can produce varying degrees of water movement. Their main shortcoming is that they only produce laminar currents. These currents for the most part can not be aimed directly onto an invertebrate because the force is so great that it can cause the invertebrate to close up its polyps. Or, in the worst case scenario, it will shear the tissue right off of the colony. However, with a little ingenuity and some additional electronics, a powerhead can be modified to produce water movement that is closer to turbulence. Coupling powerheads with a wavemaker may make it possible to get closer to producing the desired effect. Wavemakers are designed to switch powerheads on and off to try and mimic surge. Using two or more powerheads and a wavemaker may make it possible to not only produce laminar currents and surge but also turbulence as well. With the better wavemakers, it is possible to program the wavemaker to activate the powerheads randomly. This enables a random flow pattern, i.e. turbulence, to be produced at least to some degree. All that is necessary to do this is to direct the flow from the powerheads either directly onto each other or so that at least part of the flow from one is going directly into the path of the other. By having the powerheads activated randomly, the flow from the powerheads will intersect producing a random pattern of water movement. There will still be laminar currents and surge as the powerheads come on, but as these different flows collide from multiple powerheads a more random pattern of water movement with swirls and microsurges closely resembling turbulence should occur.
In addition to the powerheads, the return flow from the pump can be utilized in a number of ways. Like the powerheads, the flow from the pump can be used to produce turbulence by having the returns directed against themselves or against the flow from the powerheads. It is even better if the powerheads are on a wavemaker because their random flow will interact with the constant flow from a pump to also produce turbulence. The constant flow from the pumps can also be made more random by having it strike irregular shaped live rock before it reaches any corals. In this way the laminar nature of its flow can be reduced. Lastly, the flow from a pump can be split into numerous outflows so that small micro streams can be directed onto desired areas. There are also devices that take the flow from the pump and move it around the tank. These devices like the SCWD wavemaker can move the water from side to side between the two outlets as water moves through it. Similarly, the Sea Swirl takes water from the pump and moves it in a 90-degree arc over time. Thus the water from the pump can be directed over a much larger area than is the case when just a single outlet is employed.
There are pumps and powerheads available in virtually every size and for virtually every application. There are now even powerheads available that are designed specifically to produce water flow over a great area rather than the tight laminar flow of traditional powerheads. In similar fashion, newer, more efficient water pumps are now available that move large volumes of water quietly and efficiently. As a result, providing a reef tank with adequate water motion has never been easier.
