Not to disagree...tripper is correct...but 10x tank size is the MINIMUM recommended flow. Lots of tanks have 20x and some folks have over 30x. Another point is that the type of flow is just as important, if not more so, than the quantity. What you're looking for is turbulence. Changing, colliding currents that create swirls and surges in the tank. This is healthy for the corals. What you want to avoid is direct laminar flow. Most corals will not do well with a powerhead hammering them from one side all the time. Try to arrange your returns so that the current collides. If you use powerheads, a common arrangement is to have one on each end of the tank pointing at each other with a third in the back/center with its flow directed at an angle toward the front glass. There is no hard and fast rule for positioning returns/powerheads...just experiment so that you get turbulent currents with as few dead spots as possible.
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