If you are using a halogen lamp, you can almost certainly follow the above advice and switch to a CFL.
The specific CFL you probably want is sold by Wal-Mart. The name brand is Lights of America, which I beleive is the Wal-Mart brand.
The low wattage version of these is sold in the Wal-Mart aquarium center- Usually you can find a 10 watt version there, or maybe it was a 15.
They have higher wattage versions over in the light bulb section. The one you want specifically states on the back of the package that it has a 6200(?) Kelvin light spectrum. I might be wrong about the exact number there- it might be 6500 or 6700, but it's definately in the 6000's.
You want to avoid the GE and probably the Phillips version of the lights- They are they wrong spectrum. The Lights of America brand comes in varieties from about 10 watts to, I beleive 37 watts, while staying in the right spectrum. They make a higher wattage one, I beleive it was 40 watts, but it had a different color spectrum. If you want as much light as possible, the 25 watt one was the largest I found that fit into an aquarium fixture.
While you are in this area of Wal-Mart, look at the other CFL bulbs- These lightbulbs are more expensive then normal incandesents, but if you replace the 5 most used lightbulbs in your house with them, they will probably pay for themselves in reduced electricity bills in a few months, and last for years.
All opinions are my own. I have been wrong before, and will be again, and may be now.