The amount of light needed depends on many factors: Tank Size/depth, intended livestock and personal taste. 175 Watt works very well for shallow tanks 18 " or less with mixed reef corals and sps and clams when placed higher in the tank. 250 watt works very well for tanks up to 24" deep with mixed reef and more concentration on sps and clams. 400 watt works very well for tanks up to 48" deep and is usually used for dedicated SPS and clam tanks. For large aquariums 1000W lamps are available. It has become commonplace for one lamp to be applied to every 24 square inches in the tank. So for a tank 48 inches long, 20 inches deep, and 20 inches tall, two 250w systems would be a good choice, although more can be used (wattage & bulbs) depending on what you want. You could also use the 3-5 watts per gallon rule as shown here.

Metal halide bulbs should be replaced after 8-10 months of use or after 3000 hours of use. Although the lamp will continue to light beyond this they will have lost up to 30 percent or more of their lumen output while consuming the same amount of electricity.

If you are using a 10,000K Metal Halide bulb or higher supplementation is usually not necessary. However to achieve the best colors from fish and corals the way they would appear in the ocean you could supplement based on these guidelines:

Click here for information on Moonlights

Click on this link for information on T5 Aquarium Lighting

Click on this link for information on Mixing T5 bulbs for Aquarium use

On average a light system will increase your electricity cost from $8 to $12 per month�the exact amount depends on the size of the system and the number of hours operated. However, since these lights are so energy efficient, you are getting huge amounts of light for your money! Make sure your power circuit can handle the power draw. For safety reasons, do not exceed 75% of the rated ability of the fuse/breaker (for example: use no more than 15 amps on a 20 amp circuit). To calculate your cost, multiply the bulb wattage X hours of operation and divide by 1000. This figure is the number of kilowatt hours of electricity consumed. (Example: a 400 watt bulb running for 18 hours will use 7.2 kilowatt hours). Check your power bill for the cost of each kilowatt hour. Then multiply the number of kilowatt hours by the cost of a kilowatt hour (K/hr) to arrive at the cost per month to run the light in your area.
Click here to download the Power Cost Guide for Sunlight Supply's Aquarium Fixtures