[pogodzib]

Typically you want MH bulbs to be spaced to cover 2' of tank, which would mean 4 bulbs over a 8' tank. If you only go with 3 bulbs you'll get some visual difference in lighting in different areas of the tank. Also, how is your tank braced? Most standard 6' tanks have 2 braces. If your 8' tank has three cross braces, your bulbs may line up directly over the braces.