With the DE test, you are looking to see if the DE shoots back out of the return jet. How many days have you been keeping the chlorine at 15ppm? Have you done so consistently or is it kind of "yo-yoing" up and down? I just looked back and this thread started on the 9th. I would think your algae would be dead by now. If you aren't consistent with your chlorine levels, the algae is winning.