So I know there's a lot of you out there wondering what the best way to do this is...I contemplated getting a 3/4" square acrylic (plexiglass) bar and drilling the leds into that...but didn't want to wait, nor was the $20 price tag really worth it all that much to me. So, what I did was marked my reflector approximately every 2 inches (with the existing fluorescents and wiring/brackets in place) and then drilled 18 holes for my 5mm blue leds.
The leds I buy in bulk, I had them left over from a car stereo project, which was my main motivation for doing this. I had a 12v transformer lying around from something or another, supports up to 1 Amp of current. (Each LED draws around 20 milliampsx18 leds=360ma) I used the resistors that came with the leds (I think around 440 ohms or so) to drop the voltage, and soldered one resistor per each individual LED. At this point, I put all the leds in place and hot glued them in from above.
I then used a solid 18 gauge copper wire (I stripped a coaxial cable because I didn't have any 18/2 or thermostat wire) and clamped it to my reflector and soldered the resistor end (cathode) to one wire (+12v) and the other end (annode) to another 18 gauge copper wire for ground. I then hot glued the primary wires to the reflector lid to keep them in place, then soldered the transformer wires to the end. The end result? A 1-2 hour project that will last as long as your fish tank! I have mine on a timer so it sort of mimicks the actual moonlight cycle (comes on an hour before I turn the lights off, then completely off at 4 am.)
Here's with the lights on, shutter speed real fast to capture all the light.
I also installed a light switch into the stand for the fluorescents, with 2 separate outlets for ballasts, and one dual gang outlet for heater and filter.