According to Ron L Shimek's book How to Get There from Here, Fans should be at right angles to prevailing currents to intercept food in water. Polyp colour indicates how much food they need. Tan polyps mean they have Zooxanthellate and need some food, white polyps mean no zooxanthellae and need LOTS of food. I read somewhere also that for the white polyps an almost constant drip of food is what they require.
Douglas
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