My plaster pool is eight years old and has some rough spots. I have algae that is growing in these rough spots and is a real pain to brush off. Using my normal pool brush on the pole doesn't do it - I have to use a hand brush and scrub, scrub, scrub. Of course, getting the spots below my arm reach takes many, many dives. It seems like I can never get it all. Any tips, or do I just have to keep working it? Shock it to 20 ppm Cl (it's 30K gallons, so that means tons of bleach)? Free Cl has been running around 5 and I have been adding poly, but that doesn't do anything for these spots.