Black algae has been compared to Herpes - once you get it you always have it.
There is some truth to the statement, however, like Herpes, Black Algae can be successfully and permanently removed, with some drastic measures. It can also be controlled with less drastic measures that will keep the "virus" from spreading.
How to Permanently Remove Black Algae
Re-Plaster the Pool. Bury it alive in a concrete grave. This is the only way to permanently remove black algae; sorry. If your budget allows, replacing the pool tile (or grout) is also recommended, to remove any trace spores in the grout lines. Replace the filter media (sand, cartridge or DE grids), and replace pool stuff that has soft, semi-porous surfaces that may harbor black algae spores, including some pool floats, swim suits, pool noodles, brushes, nets, pool cleaner debris bags...
Controlling Black Algae
Most Effective Method: Drain the pool and acid wash the plaster, then pressure wash the pool, and pump out rinse water. Then bleach wash the pool . Rinse again, and then refill the pool. Replace the filter media and any semi-porous items, as described above.
Less Effective Method: Keeping the pool full of water, balance the pool chemistry to a low pH / high alkalinity condition (7.2/120 ppm), then brush the black algae heads with a steel bristled brush - vigorously. After balancing and brushing the pool, Triple Shock the pool.
The heads (or caps) of black algae have a protective coating which needs to be cut open, to allow chemicals to penetrate the organism. Actually, scraping off the heads is also recommended, using a Pool Stone or dull putty knife. You can also use a Trichlor tablet as an underwater scraper tool. Brush the pool at least twice daily during treatment, with vigor (if your arms are sore the next day, you're doing it right).
Shock the pool at night with Pool Shock, to reach at least 30 ppm of chlorine. In the morning check the pH, Alkalinity and Chlorine levels. If chlorine level is very low, around 1-3 ppm, Triple Shock the pool again that evening. Continue nightly shocking until you can test in the morning and still have 3-5 ppm of chlorine.
That's It! The tried and true methods for removing (permanently or temporarily) and controlling black algae in plaster pools. If you have specific questions, leave a comment, or call me or any one of the SPP Pool Experts at 800-983-POOL.
SPP Pool Expert