i use blue rope lights...it works great. cheap too. I wanted to use xmas lights but I did this in the summer. I figured the advantage of xmas lights would be the ability to pull bulbs for a dimmer or brighter effect...
I was under the impression that the light was just so my fish friends weren't in complete darkness. The Marine Aquarium Reference, says that even in the ocean at night there is light. Moonlight, starlight, and even light producing animals in the depths.
I usually just use the redlight thing to see in the night or quickly "flashlight" them.