Incandescent light is probably the least desirable choice for lighting your aquarium. Fluorescent lighting is far more efficient. The recommended wattage is 2 to 4 watts per gallon of water. Two 15-watt fluorescent bulbs the length of the tank is suitable for a 10-gallon tank.
Remember that too much brightness can keep shy fish in hiding. Regular fluorescent lights will need to be replaced every 6 to 8 months. Even though the light may not appear any dimmer to you, the amount of light being output declines significantly long before the tube burns out.
For more information, please read the story Lighting Your Aquarium.