The wattage of the bulb, has nothing to do with the spectrum of a bulb. A 40watt bulb can be many different spectrums. The higher the "K" rating of a bulb, the bluer it will apppear and the lower the K rating, the more yellow looking it will be. What are you planning on doing with the tank? If it is going to be a fish only tank, a normal output flourescent bulb will work. If you are going to do more(live rock, corals) then you will have to upgrade to a higher wattage light setup to get more intensity. Lower spectrum bulbs tend to promote algae growth also. I prefer 10000k bulbs with actinic blue supplements. I like the way it looks, but every one has there opinion on what looks good.