[pogodzib]

A MH bulb is much different. It needs a special ballast to run and is more efficient than Halogen bulbs. Also, halogen bulbs are generally very yellow compared to any of the bulb types used over reef tanks.

FYI - Here is a table showing the efficiency of different type of bulbs in lumens (light output) per watt:

Incandescent 18
Halogen 25
Mercury Vapor 63
Fluorescent 100
Metal Halide 125
High Pressure Sodium 140
Low Pressure Sodium 200

Since we run high powered lights over our tanks, using anything less efficient than fluorescent or MH bulbs would cost us a fortune. HPS bulbs are more efficient, but they are also very yellow.