Go for the smallest wattage "Daylight" bulb that you can find. if it is a screw in bulb, go for the 10W spiral bulb.
The color temperature you want is 6500K (6500 Kelvin) which is ideal, but anything between 5000-7000K will work.
I have two 10W GE 6500K Daylight bulbs in my 10 gallon, they work great even though the hood came with the longer tubular style bulbs.
[ame="http://www.amazon.com/gp/product/B000X487XK/ref=oh_o00_s00_i00_details"]Amazon.com: GE 85393 10-Watt DayLight CFL Spiral Light Bulb, 2-Pack: Home Improvement[/ame]
If your tank dosen't use screw in bulbs, you'll have to look around for an equivalent that will work in your fixture. Key though is the color temperature that I mentioned above.