Answer:

Black algae is typically the most difficult algae to treat. Fisrt, you have to be able to break up the outer coating on the black algae in order for the chemical treatments will work. This can be done by brushing with a stiff brush. Then there are chemicals specifically formulated to treat black algae that will work with your chlorine to treat the algae. After treatment it may be necessary to brush the bottom again to dislodge any stubborn algae. You can also improve treatment of stubborn spots by applying the algaecide directly to the affected sopts - put it on your brush before brushing.