Good question, Tanksalot. The amount of light reaching your corals varies depending upon the distance the light is away from the coral.......how far the fixture is above the tank and at what depth in your tank the coral is placed. People will tell you if you have a coral that's not responding well, move it toward or away from the light.
Light from the blue end of the spectrum travels deeper in water than the yellow, other end. Higher Kelvin(K) rating on bulbs indicates more blue spectrum. On a MH bulb, a 5500K bulb is at the yellow end while one rated 12000K or 20000K would be very blue-ish. In flourescents, there are Daylight bulbs. actinic bulbs(blue spectrum) and 50/50 or half white and half blue.
I don't have a whole lot of live rock in my tank so I can't raise my coral all the way to the surface of the water. I have my light fixture about 6 inches above the tank. The bulbs I use are a 175W 10000K MH , an actinic VHO bulb and a 50/50 VHO. I have a t. maxima clam, which requires very high intensity lighting, on my sand-bed who is very happy and growing under those lights. I also have some mushrooms that are mutipling like crazy.....and they're low light corals.
If you don't know what you want yet, look around at other tanks and see what you like before you invest alot of money in lighting that you're not going to be satisfied with.
Check out the Photography section of this board for lots of pictures of other tanks. If you see one you like, ask the poster about his/her lighting........
Jim
