[Al Kaseltzer]

Had to delurk as I considered this idea a while back. The important factor your missing is the wavelength of the UV light. check out this chart:

http://www.priva.ca/images/vialux-action-spectrum.gif

Now because any water cooling system is going to be cycling water through the UV over and over, it doesn't have to be as effective as a UV disinfector where the water only goes through once before you drink it. According to the chart though, you'll still have to be somewhere under 300 nm.

From this site:

With a peak wavelength of 350nm, this is the shortest wavelength UV LED I've heard of in current production.

So UV LEDs won't work very well for disinfecting. One other interesting thing I learned while browsing around that site:

the 20-LED NUV Adapter radiates intense near-ultraviolet light at right around 395nm where most fluorescent stuff glows.

Which tells us the probable wavelength of most decorative UV sources, like CCFLs meant for modding, or black light bulbs meant to light up your flourescent posters.

So if you want to try any other UV source that's not designed for disinfecting, make sure you find out the wavelength first.

--
Al Kaseltzer