I am setting up a 30 gal tank, and plan to hang my lights about 12" to 15" over the tank. Will the distance above the tank effect the "watts per gallon" rule.

 

Yes I would think so. In the light penetration that's going to suffer the most. What type of lighting did you have in mind. Some 96w cf might do the trick but 2x36w that I use on my 30gal all thou work find on top I don't think they would work for foreground plants raises 12" over the top.

Hawk

 

I assume you are planning on using HID pendants. You have to know the dispersion angle for your pendants, then do some ray tracing through the water to get the optimum height. Don't forget to add in the refraction at the water's surface. Otherwise (if you hate to plan stuff) move them up and down over the tank to find the optimum height by trial and error.