Matt, the film on the media is brown. Areas that get more light have some green algal growth but not much. The shower is in the shade of a pair of redwood trees, though the southern end of it does get some sun. I have a sheet of matting on top of the top tray media to help keep out the big pieces that get through the skimmer baskets, as well as leaf drop from the trees. So, no, light is not completely blocked out, but it's mostly shaded. I will say I've been surprised that I haven't seen more algae.
When I had the Stak-Paks, light was almost completely blocked, other than the cutouts for the spraybar in the lid. But that blocked air as well, so I don't have the answer to that conundrum. Is it even possible to block light but not air? If nitrosomonas are light-avoiding, why do shower filters all seem to eliminate nitrate? Most ponders don't have their showers in the dark. wait, lemme rephrase