I would actually advise against either of the options offered so far. Neither the eBay or diy units suggested have what would be considered moisture resistant end caps. I'd be hesitant to install either of them over a tank full of water long term.
I'd recommend something with water resistant/proof endcaps such as the Hagen GLO T5 retrofit kit. There are others I'm sure. Looks to me like for around $200 you could build a pretty good system, here's a link to get you started, just the first thing I came up with when I googled Hagen GLO T5HO retrofit:
http://www.marineandreef.com/Hagen_G...p/rhg11555.htm
I ended up wiring up my own DIY system using Fulham Workhorse 8 ballasts stripped out of an older fixture. Purchased waterproof endcaps, individual reflectors and voila. But the Hagen kits seem to come with most everything you need to get you set up fairly quickly.