Black Algae looks like small black spots on the plaster with a slimey outer coating. Generally, black algae grows in colonies, but also individually on the pool walls, floor and steps.Black algae will grow due to alkaline water (high pH and alkalinity), low chlorine, high nitrogen levels (not “shocking” the pool enough), and low circulation (run time).For visible black algae growth (Plaster pools only):Adjust pH to 7.0-7.2 ppm – Alkalinity to 80 ppmBrush black algae with stainless steel pool brush (to remove protective outer skin), turn circulation pump OFF, let water settle and add 2 lbs. Black Out (granular trichlor) per 10,000 gallons. Broadcast Black Out directly on algae spots and let sit for 8-10 hours.Turn ON circulation pump, brush pool with stainless steel pool brush and let pump run 24 hours.Shock pool once a week during the swim season and test pool water weekly.As always, call or stop by the store with any questions.