Lighting calculations

[WWWD]

It also depends on the depth of your tank, deeper you go the less light penetrates below. Also keep in mind that the useful par value in CF lights are pretty much used up within the first 6 months of the bulbs usage. While they might fire and even look bright they have the same effect as floresent office bulb in terms light your corals need to grow.

I think others have mentioned that metal halides are the most cost effect means of lighting most tanks, you should consider that before you get locking into a lifetime of buying CF bulbs and wishing you hadn't.