How to get rid of black algae in a pool
What is black algae?
A black algae is a thread-like growth on the rough surfaces of the swimming pool. Black algae will appear as dark black slippery spots on pool surfaces (walls and floor), and also forms a protective coating.
Black algae forms a layered structure where the first layer (which chlorine may kill) protects under layers from further destruction. Black algae are similar to the black algae that is found on bathroom shower tiles and in silicone seams near the bath. This form of algae is very slow growing but very hardy. It is extremely chlorine resistant. This is why you will need to, in addition to shocking the pool, use a silver based algaecide
Some other articles I have read claim that black algae is very difficult to get rid of. In my experience, this is not true even with a severe break out. The only difficult part is making sure you brush all affected areas in the pool.
Black Algae Treatment
I mentioned brushing. Before adding any chemicals, you will need to brush the entire pool (all black algae spots) with a STEEL bristle brush. Your nylon wall brush will not work since the black algae has a protective coating that needs to be brushed off and exposed. NOTE: If you have a vinyl lined pool you will NOT use a wire brush as this can tear the liner
Once the head of the all black algae have been brushed and exposed you will shock or super-chlorinate the pool using granular chlorine (Calcium hypo-chloride). TriChlor shock such as the kind found at many big box retailers may also be used. Make sure you raise the chlorine to at least 15 ppm (parts per million) for 24 hours. Be sure to add 8 ounces of a silver based algeacide per 10,000 gallons of water immediately after shocking the pool.
After 24 hours the black algae may not be completely eliminated, but will take a few days for the black algae treatment to work along with continued brushing of remaining visibly affected areas. I recommend a quart of black algae algeacide as a precaution and for regular maintenance doses. As mentioned above, A silver algeacide, such as "Silvertrine" works the best. Don't let the color of the algeacide scare you as it looks nearly black when adding it to the pool. Make sure to add this through the skimmer.