 Re: Simple refuge without sand?
The Macro Algae will grow on the rock or on sand and some will grow without either. The Caulerpa Algae will grow roots after a week or two and try to latch on to something. The chaetomorhpha algae needs nothing to grow on it just spreads itself out. Here is a link to a good sight on Macro's I do not endorse them but have purchased from them and they treated me right. http://www.liveplants.gulf-coast.net/
I do not believe you need too many additives as they filter the water and take what they need. Read the requirements section under the algae pictures.
I would go with a regular grow light on the sump 75 watts and run it 24 x 7 or the reverse lighting of your main tank. I hope this helps in your decisions.
Jim
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90 Gallon glass, 135#'s of rock and 2" sand. 2-250w 20k XM metal halides. 2-95 watt actinic VHO, 4 powerheads, Coralife skimmer. Maroon Gold Striped Clown, Blue Chromis, Kole Tang, Pygmy Angel, Serpent Star, Brittle Star,Gorgonians, Mushrooms, Yellow Polyps, Turbo Snails & Blue Legged Hermits. 20 gal sump with Caulerpa, Mag 7 return pump. | 
