Update: last night I brought the level up to 15ppm. As of about 2 hours after dawn this morning, I had lost 2ppm (reading was 13). No growth showing on walls. A little less cloudy, but there is a large buildup of brown-ish "dust" in the crevices of the floor.

On a related note: anyone have advice on whether I should put some DE in the skimmer basket to test the filter and to help the filter catch smaller particles? I have zeo sand in my filter.