Just now in the beginning stages of shocking a green, algae infested in ground fiberglass pool. I used a regular pool brush and did a decent job of knocking off some of the algae stuck to the walls. However, the circular looking spots act as if they don't want to come off with just the swipe of a regular pool brush. I can rub them off with my finger fairly easily, but I obviously can't do that for the entire pool. Any ideas...do I need one of those steel bristle brushes? Or will they break up easier after I shock for a while? I can't really get too agressive with anything since I recently had back surgery. Thanks in advance for your help.