I took out my 2800 psi pressure washer, put on the 15 degree tip, and pressure washed the walls. (yes, the pool was still full of water, so you just hold the tip about 1 inch fro wall) and it blew all the black algae off the wall and out of the cracks, then I shocked the hell out of the pool for about a week, and man o man, it made a huge difference.