If you go with a white LED, here is a trick I used:
In order to get a "fuller" coverage of light in my tank (now this was for a blue LED for nightime viewing) I actually inverted the light so that it shone up and into the hood of my tank. This diffused the light enough to actually light the entire tank without losing the "cool-moonlight-evening" look. Also made for a great shimmer.
If you just point a LED down, you tend to get an intense cone of (enter color) light straight down and have dark spots on the periphery.
Ooh blah, blah what-ever blah, blah... sorry, got a little long winded. -FM.