Black algae has a way of making even well‑kept pools look defeated. We dig into what it really is—cyanobacteria protected by a stubborn biofilm—why it laughs at normal chlorine levels, and how it burrows into plaster and Pebble Tec where a quick brush won’t touch the root. If you’ve spotted dime‑size black dots that keep coming back, this is your field guide to taking control without trashing your finish.

We share a step‑by‑step plan that starts with breaking the biofilm using the right tools: a small stainless steel brush for plaster and Pebble Tec, nylon on fiberglass, plus a stain eraser for tight spots. Then we layer in chemistry the smart way. Chelated copper (like PoolRx) provides steady background pressure, while targeted oxidizers finish the job. You’ll learn when granular trichlor is the fastest hammer on plaster, why it can burn colored finishes, and how to use calcium hypochlorite as a safer alternative—especially in cooler water where it lingers on the spot. We explain the tablet‑chunk method for cal hypo, why dichlor dissolves too fast, and how to broadcast without clumping to avoid stains.

For extreme infestations, we outline when a drain and chlorine wash beats an acid wash, the real‑world risks to aging plaster, and how commercial pools benefit from a full reset that also addresses high cyanuric acid. Prevention gets equal airtime: keep free chlorine from crashing to zero, hold 5 ppm or more during recovery, maintain chelated copper to deter regrowth, and stop cross‑contamination by dedicating brushes and nets to infected pools. We also highlight the early signs pros catch—pencil‑eraser dots on steps and crevices—so you can act before colonies spread.

• black algae as cyanobacteria with protective biofilm
• why worn plaster and crevices worsen colonization
• cross‑contamination from shared tools and how to avoid it
• copper as steady background pressure and limits
• trichlor granular for plaster and risks on colored finishes
• cal hypo as safer Pebble Tec option, tablet chunk method
• when to shock to 30–50 ppm and why

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