Thank you again for your helpful advice. After a couple of weeks of faithful scrubbing and superchlorinating every day, we kept noticing new outbreaks of the algae. :-( Took a water sample to the local pool store and our salt tested at 6100, but chlorine was 0. Oops! Our salt display had consistently read 0 for a couple weeks. From that, we figured out that the AquaTech cell was bad, was not reading the salt, and was not producing chlorine. So yesterday we drained half the pool and refilled it, and replaced the cell. In the process DH turned the handle from "waste" to "filter" without turning the filter off and cracked the casing and broke the handle off. :-( So we are unable to run the filter or heater at all till Monday when we can hopefully get the parts replaced, get the water properly balanced, and get the black algae under control once and for all.

Anyway, I was so frustrated with the black algae situation that I did buy some algaecide at Leslie's and it surprisingly worked quite well. The instructions were very vague, but this is what we did. As I mentioned we had already been scrubbing hard and faithfully once or twice a day. We gave it another good, thorough scrubbing and turned off the filter. Then we dilluted the solution and poured it around the perimeter of the pool, and turned on the filter a couple hours later. The next morning when we drained half the pool most of the algae was gone.

Thanks again for your helpful advice!