Black algae rarely occurs in swimming pools, but when it does, it will take a minimum of 10 days to clean the pool walls, floors and water to get rid of it. Black algae appears in a pool as tiny black specks that resemble dirt particles. This type of algae can start in any part of the pool, unlike mustard or green alga, which usually starts in the shallow end of the pool near the corners.
Inspect the entire pool in corners, open areas, around stairs, ladders, walls and the floor in the deep and shallow ends of the pool to find the areas where black algae is starting. The best way to do this is by getting in the water with goggles on and inspecting the pool thoroughly under water. Keep the filter operating as normal while ridding the pool of black algae.
Brush the black algae spots within a fiberglass, shotcrete, gunite or concrete pool with a stiff wire-bristle brush. You must rub hard on the black algae spots because the alga has a strong outer shell. If you have a vinyl liner, use the telescopic pole attached to a soft-bristle pool brush, but press down hard on the spots to try and break away the hard shell of the black algae. Brushing should be done twice during the day and once at night.
Add shock to the pool water at night after the last brushing. Use 2 1/2 lbs. of shock per 10,000 gallons of water if you have a vinyl liner or 2 lbs. of shock per 10,000 gallons of water if you have a fiberglass, shotcrete, gunite or concrete pool. Walk around the entire pool while slowly pouring the shock into the water.
Brush the pool twice the next day. After you finish the last brushing for the day, add metallic algaecide, which contains copper and helps fight against black algae. Follow the directions on the container to add the correct amount to the pool water.
Brush the pool once a day on days three, four and five. On day six, brush the pool one more time. In the evening, as the sun sets, add pool shock at 1 lb. per 10,000 gallons of water. Walk around the pool while slowly pouring the shock into the pool.
Brush the entire pool again on days seven and eight. On day nine, brush the pool and add the recommended amount of metal sequestering agent to the pool water to counteract the copper in the pool from the metallic algaecide. If you skip this step, your pool can become stained and any metallic fixtures will corrode due to the copper in the pool. The metal sequestering agent removes any copper that is left in the pool.
Monitor the water for any new black algae in the next few days. Assuming that you have a clean pool free of black algae, you should now test the pool water using the pool test kit and adjust the chlorine, pH, total alkalinity and calcium levels as needed.