How much light does a 100 gallon aquarium need?

This will depend entirely on what you would like to keep; for fish only, a small amount of light will do, basically whatever looks good to you. If you are going to keep photosynthetic organisms such as plants or corals, you will need specially designed lights. For an aquarium that large (6 feet long) you are looking at somwhere between $150-$3000 depending on the above.