Another factor to consider in conjunction with color is par value. A 10K bulb will give you greater par (light output) than a 20K. Of course you give up some of the colors by doing so. Par is an important factor for coral growth. While the greens, blues, and purples are great under 20K, you won't see the oranges and reds like you will under a 10K.

I've run everything from 6.5K to 14K MH's and have settled on 10K with VHO actinic supplementing them. I think that gives me the best balance. I think the answer is that you will never get a good balance running only one bulb. My current plan is to add a 20K MH on my next lighting remodel. Another problem with 20K MH is that they don't put out the 420 nm light that corals need. That's one advantage of the VHO over PC's and MH's.

On my 125 gal. I'm running 2 X 250 watt 10K and 2 X 140 watt 20K actinic VHO's. The only problem I currently have is that the 10K's partially wash out the actinics. That's why I am thinking about adding the 20K.

Gary