Some could argue that water movement is the most important thing that our friends in the ocean need to survive. Other may say it is solar energy. Either way, water current IS very important. Without water current most invertebrates would die and most vertibrates would be significantly effected. Water movement brings food and necessary chemicals near and takes waste and unwanted chemicals away. It dictates what shapes the reefs will take. In some cases it plays a key role the reproductive chain. On the reef, this water movement is caused by several factors.
Wind, Waves, Tides, and Temperature
Wind is the basic cause of waves. As the wind blows across the water's surface, it pushes the water along with it, creating waves. The gravitational pull on the water by the earth and moon cause what we call tides. Waves and tides are affected by the temperature of the water and the surrounding air. All of these factors cause the water movement to have different intensities and dynamics. Large waves crash as the they get closer to the shore with great force. Lagoons don't have much current at all on average. What does all this mean?
Different specimen require or enjoy different current speeds. On live reefs, current speeds have been measured ranging from 10 feet per second to less than an inch per second. Some corals live deep below the surface and receive very little current where others live closer to the surface and thrive in more current. It is recommended that each person research the exact reef conditions that their specimens lived in and try and replicate that as close as possible for the highest rate of success. As a very weak general rule, it can be said that the more light a specimen requires, then the more current it probably is accustomed to. Now this is not always the case as can be plainly seen when we discuss vertebrates who for the most part don't 'require' light. But it is a nice guide line to start with when considering invertebrates.
Variation plays a big role
Water current is rarely a constant thing. On the reef the currents are stronger at one part of the day and less at other times. Waves are just that....waves. They are not a never ending flow of consistant pressure over time. Using power heads that cycle on and off or that oscillate will help mimic that behavior. When calcualting current, be sure to average the ammount over time. If the desired current is 20 inches per second, then a flow rate of 40 inces per second that is off the same amount of time that it is on is a good way to look at it.
Spend time planning for current
Current is as important as light is in your home aquarium, if not more important. Invertebrates need a fresh supply of nutrients and chemicals like calcium to continually be presented to them regularly in order to thrive. One of the biggest mistakes that people make is not producing enough current in ALL areas of their tank. This includes behind the rocks and in the corners. Stagnant water is a sure killer to most of the items you would may one day wish to grace your show piece. Most corals need current to perform a process called calcification. To do our part to promote that requires about 7-10 inches per second of current. Spend some time calculating what pumps and plumbing will be needed to give your inhabitants the water motion that need.