6.5k and 20k

[sunkool]

If 6.5k is for fast growth and 20k is for best color why not run both? Would you get the fast growth along with the full color or would the 20k over power the 6.5k?

[JetCat USA]

just the opposite, the 6500K would over power and still yellow out the 20KK, not what I'd recommend for any display tank.

[IslandCrow]

You can certainly run both. The reason you might not is if you're going strictly for growth, why not get twice the light in that spectrum with 2 6500K bulbs instead of one 6500K and one 20000K. I don't see it so much as one overpowering the other. The resultant light will just appear somewhere inbetween the color of the two bulbs. It also depends on what type of bulbs you're talking about. With metal halide, since their light is fairly tightly focused compared to florescent, you'll have the area under the 20,000K bulb looking pretty blue and the area under the 6500K bulb looking pretty yellow.