If you see black spots growing in your pool, it is a sign of the beginning of black algae and you will want to treat it immediately. The longer you let it go, the harder it will be to eliminate it. Black algae generally penetrate porous pool surfaces like concrete and gunite. Once these surfaces are infested with black algae, it will take time and persistence to eliminate it. Black algae are generally not an issue with smooth-surfaced pools like fiberglass and vinyl. This guide provides a step-by-step procedure for eliminating black algae.
ASSEMBLY TOOLS AND CHEMICALS - Buy the required tools and chemicals ahead of time to have them on hand for this procedure. If you have a concrete or gunite pool, you will need to buy a stiff brush like one with stainless steel bristles. The chemicals that you will need to buy are shock treatments and an algaecide.
BRUSH POOL - Next you need to brush off the black spots vigorously with as stiff a brush as you can use. If you have a concrete or gunite pool, use a stainless steel bristle brush. If you are one of the few that have black algae in a pool with a smooth surface, use the softer nylon bristle brush. Black algae have a protective cover over the algae roots that penetrate the porous surface of the pool. This barrier has to be removed so that the chemicals can get to all of the algae. Brush the algae spots three to four times a day for the duration of the treatment. Pay special attention to the pool's corners and around fixtures like the pool ladder.
ADD GRANULAR CHLORINE - If you have a pool with a lightly colored surface, sprinkle Granular Chlorine over all the areas where it may settle on or near the black algae. Do not use Granular Chlorine in pools with dark surfaces as it may bleach the surface.
RUN PUMP - If you have a pool with a light finish, leave the pump off for 24 hours to let the treatment set in. Then, after those 24 hours, run the pump an additional 24 hours the next day. For a pool with a darker finish, run the pump for 24 hours after adding the chemicals to prevent bleaching. During this 24-hour/48-hour period, continue to brush three to four times a day where the black algae spots were.
RE-SHOCK POOL - If there is any indication of the black algae reappearing after three days, shock your pool again with a normal dosage - one pound per 10 000 gallons of water.Continue to brush where the spots were for two to three more days. Don't skip brushing. It is a key part of the treatment.
MAINTAIN POOL CHEMISTRY - Once you have had black algae, it has a tendency to reoccur. It is important that you maintain your pool's chemistry and that you brush down the pool walls and floor once to twice a week. Note: Due to the heavy chlorine treatment, the chlorine level may remain high for up to two weeks.