On day 7, I stirred the starter, discarded half (around 4 ounces), and fed it 1/2 cup white-wheat flour and 1/4 cup filtered water, mixing it up so there was no dry flour left.
By the seventh day, the sourdough starter pretty much looked like it had regressed back to day 1. I decided to try switching to white-wheat flour (not whole wheat yet, since I had a hard time finding at the store).