You are asking excellent questions.
We call it the HD Special because it's the cheapest light that can be bought and still grow coral and algae. It's a Daylight twist PC bulb in a clip-on cone reflector(see pic at the end). I mentioned it only because it's good to know about. To some people, the reflectors don't look good for a visible display but many of us use this light for our Refugiums. Several hobbyists I know do use those for their display, also adding those Blue twist PC bulbs to add more blue light. Coral must have blue because that is the only color that reaches them. I've posted a Jacques Cousteau picture series before. I will post it again here.
Sunlight above the narrow blue range of the spectrum is stopped by the water at depths below 15 feet. Blue light shines very deep. At 15-60 feet is where most photosynthetic coral grows in nature. When a tank is placed near a window, the indirect sunlight still has much of the blue spectrum which greatly benefits coral and all organisms in the tank.
I have sunburned and even killed coral by all day exposure to direct sunlight in 1 foot deep tanks.
Aloha
Edited by Mark Peterson - May 31 2014 at 9:07am