In most cases, fans are required to provide for better air circulation around the lights of the tank. The idea is to not have any hot air from the bulbs hanging around the top of your tank. All of my PC canopies have small computer-type fans. Most are 12V DC and 1-3 watts. You can get them for $12-20.
Depending on lighting and setup, you may not need fans. If you use metal halide...you need fans, PC or VHO....you may need them depending on if you have adaquate circulation and size of tank. If you use NO bulbs...you should not need fans.