I built a simple system for blue party bulbs to give my tank a moonlight type effect and it only cost around $15. I just went and bought 2 light bulb fixtures that had screws on the side for wiring an electrical cord and two holes for mounting to wood or another surface. I also purchased some replacement electrical cord and a plug. I wired the two light bulb holders (for lack of a technical term) together and on one of the also wired the rest of the cable for a plug. Something like as follows
light bulb -- cable -- light bulb -- plug
I then mounted the light bulb harnesses on either side of my tank canopy, plugged it into a timer and instantly had a little moonlight that comes on for a few hours. It gives everything time to still locate food, get a hiding place or whatever before the entire tank goes dark for a few hours. Then in the morning, the light from the windows gives it sort of a dawn effect. I think I used 15 watt or maybe 7 watt little party bulbs. It gives it some nice light to see certain parts of the tank, however I do sometimes still need to use a flashlight for the darker areas between rocks.
--Andy