One of the larger spots is virtually gone. I just notice a mustard color spot in the deep-end. I am still scrubbing the smaller black spots with the metal brush but it does not seem to be making any difference. I treated the pool with the algae killer Friday. Can I treat it again? Should I clean the filters also? The pool is made from gunite and I'm not sure what the plaster/paste that they finished the pool with is. Thank you for your response.