If it's for your 20 gal
, the standard light will most likely not be enough to keep the coralline and other low light hitchikers that come with the rock happy. If the rock is primarily for filtration benefits, I would instead suggest you look into base rock and worry about the light later. The filtration aspects of the rock are not light dependant.
If you want the coralline to grow, I would suggest at single 96w PC
powerquad or possibley a 2x55w PC
. It's not so much the intensity but the spectrum that's important for coralline though. 2+ watts/gal
should suffice for that, primarily actinic.