The spectrum of Actincs ARE in the photosynthetic range of your corals, and broaden the spectrum of the "Peak" that Metal Halides put out. The issue is that most people use low watt Actinics, that won't show a noticable help with your coral growing. But as for flat out "They don't help" That is wrong, they do, just not as much as the MH. I turn mine on as a dawn/dusk for my tank for an hour before/after the halides, all my coral wakes up, and has incredible polyp extension during these times. When i was turning them on at the same time as my halides i would not get this coral feeding time.
refer to :
http://www.advancedaquarist.com/2009/3/review
