Read this today thought it was intresting. Makes me wonder why you cant just rack up any high intensity lighting no matter what the k value and be good to go. Intresting reading if you have the time.
"This article presented a comparison of natural underwater light (based on specific data from natural waters) to the light over our aquariums. While there are variations to be expected in natural waters, the general shape of the spectral distribution underwater will be quite similar. Clearly there is a wide discrepancy between the spectral irradiance provided by MH and the natural under water light field. We know from experience that we can grow coral under all the 3 major classes of metal halide lamps 6500K, 10000K and 20000K, so the corals must either adapt to the spectrum or ignore the spectral quality. Unfortunately I do not have any definitive answers to this, hopefully further research will be able to provide more definitive answers."
http://www.advancedaquarist.com/2005/8/aafeature
"This article presented a comparison of natural underwater light (based on specific data from natural waters) to the light over our aquariums. While there are variations to be expected in natural waters, the general shape of the spectral distribution underwater will be quite similar. Clearly there is a wide discrepancy between the spectral irradiance provided by MH and the natural under water light field. We know from experience that we can grow coral under all the 3 major classes of metal halide lamps 6500K, 10000K and 20000K, so the corals must either adapt to the spectrum or ignore the spectral quality. Unfortunately I do not have any definitive answers to this, hopefully further research will be able to provide more definitive answers."
http://www.advancedaquarist.com/2005/8/aafeature