full spectrum will give nicer color and definitely grow sps. But i reccomend 1 fixture for every 2x2 section as the d120s have lenses that focus the light so it will not spread near enough for a 4 footer. Or if you have your mind set on 1 fixture and are happy with that one you should do the same again as i wouldn't put a d120 on anything bigger then 30" (and that is with stripping the lenses off or changing them to 120 degrees).
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Nick
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