Today was the big day! I scraped off the green mess on my glass. This is about 20 days from when I last scraped it, and 18-19 days since I first saw that my last scraping did no good at all. The GDA had gotten considerably thinner, so much so that I could see the plants through it, so I had to assume that I was in the adult stage of the algae growth cycle.
In addition to scraping the glass on two sides - I left two alone just to see what happens - I also made major changes to my filtration and CO2 injection. I got a Maxijet 600 and a sponge filter for it, installed it upside down with the CO2 tube sticking down into the sponge support grid. Then I removed my Fluval filter entirely, removed the little minijet pump that I had used for CO2 mist, and am now using the powerhead as my only filtration. That removed the inlet and outlet tubes for the Fluval, and leaves me with no place where debris can clog up the suction, a possible factor in other algae attacks. So far the CO2 mist looks great and the water cleaned up very nicely.
Of course, now I will be walking on eggs for a week waiting to see if the GDA returns to the glass. Today was the first I have had a good look at my plants, shrimp and fish in over two weeks!