If you use the diffuser grating, you can paint the top and bottom so that the only areas of the screen that get light are the brightly lit areas in the middle, which would resolve SM's concern. Also, it would stop algae growing on the airstone.
You might also consider putting something behind the screen to block light from going into the sump, as you'll likely get a lot of algae growing in there eventually.
What are your N and P measurements currently?