I have learned from a very painfull algae experience that although I love the colour of 18000k bulbs, and actinic bulbs, they certainly do encourage algae in "certain" configurations. In my low light (under 2wpg) tanks I have 18000k bulbs with no problem, but in tanks that are high light, or tall tanks (over 2 feet tall) then I seem to have horrendous problems with staghorn algae or diatoms.
I recently tested this theory by using different bulbs over the 90 gallon tank. The results were simply astounding. Under the 18000K + Actinic bulb was a stripe of Staghorn algae. It didnt grow anywhere else, just under these bulbs. The rest of the bulbs ranged from 2700k-5000k and had no problems at all!