yea a lot of people do that for astetics and it works well for less light demanding critters, but it is not generally recomended for higher light organisms like clams and sps's. Sure a lot of people with 400W halides run 20,000K but thats because they have plenty of light and get the intensity needed for photosynthesis through wattage and the intensity of MH. When your talking flourecent lighting you get more PAR and "brightness" out of 10K's, actinic whites, and 6500K type bulbs and if you want clams you may have to switch back to 50-50. I don't have a specific thread to point you too, but I have read this advise over and over from the experts like Calfo and Fenner on WWM as well as others. Some even recomend that if you use the old 5W per gallon rule you have to exclude the actinics from the equation. Now I don't know if I would go that far, because some photosynthetic pigments actually do best at converting some of the lower wavelengths (about 450nm) But on this sort of advise I waited till I had halides to try clams, of course there are others like mikeguerrero who have success with flourecent lights so for every "rule" there are exceptions, but remember he does have 9W per gallon which is pretty high and a very shallow tank.