DE Bulbs: In your opinion, (and experience)

[Fish]

A 20k will usually provide a more asthetically pleasing colour, but a comparable 10k bulb will be WAY brighter.

It depends if you are asking about what gives the more 'intense' colour or the more 'intense' lighting. If you are talking about how strong the lighting is, you are probably talking about PAR. A 10k bulb gives off more PAR than a 20k bulb will. To get that nice blue color on a higher k bulb, you are giving up radiation that the corals could be using for photosynthisis. Even if the tank doesn't look dimmer to the naked eye, a quantum meter can detect the difference. As an example, the 250w xm10k bulb on an icecap ballast has a PAR reading of 640. On the same ballast, the xm20k was measured at 270 - less than half the amount of photosynthetically available radiation!
I once almost fried my corals when my 70w 20k burned out and I put a 10k iwasaki in as a replacement. Now I have another 20k and I use that because I prefer the colour and I don't need a lot to light my 12" cube.


This thread on RC is what I used to make my 250w lighting decisions: http://www.reefcentral.com/forums/sh...hreadid=254667

Sanjay's site is good too: http://www.reeflightinginfo.arvixe.com/index.htm

HTH,

Chad