most of the "easy" LED power supplies have a minimum output voltage, so if you wanted to run only one LED, you would have to put some resistors in line.
for example the LPC-35-700:
http://www.rapidled.com/servlet/the-...drivers/Detail can run 3-12 of the 3.7Vf LEDs. Im not in the state of mind to do the math for you here, but you'd have to put some decent sized resistors in line if you wanted to just run one LED with it.
My suggestion.
Get that driver, get one cree XP-G cool white, get one or two cree XR-E royal blues, pack the 3 of them super tightly together on some sort of heat sink and go from there. The XP-G is about 30% brighter than the XR-E so you'll get about a 20k look (since you're displaying stuff this might be best anyway).. A single cool white LED over the tank will give you an extremely yellow look and I guarantee you wont like it.
If you only end up doing two, make sure you wire a proper resister inline that has a capacity of at least 5Watts, and I think it should be something like 5 ohms?? not positive.. vicodin from this tooth pull is making me hazy.