I have a pebble tech pool. Was salt, but switched to chlorine. Had an algae problem 2 weeks ago, but got everything dialed in now and the water is testing well. However, I have some dark spots left at the bottom of my pool from the algae I assume? I have about 1/2 dozen that are about 1-2 feet in diameter. The pool vac won't clean it and neither will the pool brush. Is it black algae? What if I put on a glove, swam to the bottom of the pool and scrubbed the spots with a 3 inch chlorine tab? If I have to drain and fill, I'll just wait another couple years. Thanks for any advice. AZ, 15000 Gallons, Pebbletech, Chlorine, DE Filter, Hayward Floor Vac.