The following is from a nano lighting article from http://www.reefkeepers.org/faq/cache/9.html
Recent conversations with Jimmy Chen, Ron Shimek and others concerning Craig Bingman’s suggestion of scrapping the old “watts-per-gallon” rule in favor of “watts-per-square-foot-of-tank-surface-area” look very promising. Initial comparisons seem to definitely support watts/ft2 as a more accurate measurement of a tank’s requirements. It has been suggested that the target number one should shoot for on a 24” deep tank is 100w/ft2; my own best guess is that anywhere from 55-75w/ft2 would be perfect on a tank 12” deep or less. You may wish to bear this in mind when figuring out how much light to use on your tanks.
There was another article from Ron Shimek that I can't find at the moment that stated something like " if you have enough intensity, spectrum doesn't matter". With enough bright light, your creatures will get what the require. That's not to say that a lot of light in the "wrong" spectrum won't produce undesirable results (algae, poor aesthetics). However, 6500K is considered a daylight bulb and IMO
fully suitable for a reef tank. I would not hesitate to use 6500K. Sounds like 180W is about right.