6500K is the closest to natural sun light and about 15-20 yrs ago they were just about the only "reef" mh
bulb you could get. I used them for many, many years. The thing about that temp bulb is it is somewhat yellow and you need heavy actinic supplement, unless of course you don't mind the color. A far as growth, I never really noticed a difference between that and the 15K's I use now, but I used more actinic with the 6500 than I do now with the 15K's...so I guess it sort of balanced out.