Join Date: Jul 2010
Location: Outside Philadelphia, PA
Consider that a 29 gal tank has a bottom surface area of 360 sq/in and a 40b has a bottom surface area of 648 sq/in, you can see that you have the same light trying to light up almost twice the surface area. I would expect the 40 gal to look dim compared to the 29 gal.
Depending upon what you want to keep in the tank, consider upgrading the lighting. You could either add an additional fixture, or replace the whole thing.