Usually the hood. Bulbs don;t make a difference so much with flourescent lights. The ballast controls the wattage of bulb you can choose. Check out AH supply. They have Do-It-Yourself setups that you can use in your existing hood. Their reflectors are generally well respected in the hobby. Now, if your aquarium came with a top that had a clear glasss strip in the middle that your light just sits on, you can purchase a strip light and not have to do it DIY if you don't want to.