aquarium water turnover every 6 minutes. This is a general rule with many exceptions. Some corals, such as mushroom corals and polyp corals, require very little flow to thrive. Conversely, large-polyp stony corals such as brain coral, bubble coral, elegance coral, cup coral, torch coral, and trumpet coral require moderate amounts of flow, and small polyp stony corals such as Acropora, Montipora, Porites, and Pocillopora require high, turbulent, conditions, which imitates breaking waves in shallow water near the tip of the reef. The directions which water pumps are pointed within an aquarium will have a large effect on flow speeds. Many corals will gradually move themselves to a different area of the tank if the water movement in its current area is not satisfactory. "Since flow speed is the critical measure for determining the rate of gas exchange, turnover does little to convey how fast a coral will respire and photosynthesize." Reef ready tanks obtain at least a portion of the required water motion from the pump that returns water from the sump. This flow usually is augmented by other strategies. A popular strategy is placement within the display tank of multiple powerheads. Powerheads are simply small submersible water pumps that produce a laminar or narrow, unidirectional water stream. If the presence of the powerhead in the tank does not fit with the aesthetics of the display, small holes may be drilled in an overflow of a tank and the bulk of the powerhead can be hidden, leaving only the small funnel spout visible in the tank. The pumps may be alternately switched on and off using a wave timer and aimed at one another or at the aquarium glass to create turbulent flow in the tank. Drawbacks to the use of these powerheads include their capacity to clutter the display tank, propensity for excess heat production, and the laminar quality of water flow often produced. Another method is the closed loop in which water is pulled from the main tank into a pump which returns the water back into the aquarium via one or more returns to create water turbulence. Newer submersible propeller pumps are gaining popularity and are able to generate large volumes of turbulent water flow without the intensely directed laminar force of a power head. Propeller pumps are more energy-efficient than powerheads, but require a higher initial investment. Another recent method is the gyre tank. A