The Dark Side of Pools: Black Algae Survival Tactics

Black algae has a reputation that matches its look: stubborn, slow to die, and quick to return when ignored. Despite the name, it isn’t a true algae but a cyanobacteria wrapped in a tough biofilm that shrugs off normal chlorine levels. That shield is why you can hold 5 ppm free chlorine and still see the same dark, coin‑sized spots stare back from plaster or Pebble Tec. The organism wedges into tiny pits and micro‑cracks, anchoring itself deep enough that a casual brush only takes off the “head” and leaves the root behind. If your pool surface is aging, etched, or cracked, expect more colonization and more frustration. Success starts with understanding the biology, then matching it with treatments that expose the cells and keep pressure on them for long enough to make a real dent.

The first critical move is mechanical: break the biofilm. A small 5‑inch stainless steel brush gives better leverage than a wide one, letting you attack each spot with force. On fiberglass, avoid stainless and use a firm nylon brush to protect the gel coat. A stain eraser attachment can help where brushes miss, especially along steps and tight curves. Don’t rely on brushing alone; it merely spreads debris if you don’t immediately follow with a targeted sanitizer. And never shuttle the same brush and leaf rake between pools with active black algae; you’re likely transferring the problem. Many pros leave a dedicated brush and net at the affected site to reduce cross‑contamination and save their route from a cascade of callbacks.

Chemically, think combination and contact time. Chelated copper, such as from a PoolRx unit or a quality copper algaecide, creates steady background pressure that weakens cyanobacteria over days to weeks. It’s not a miracle cure, and heavy infestations will outpace copper alone, but it’s a strong pillar in a layered approach. For topical kill, granular trichlor is brutally effective on plaster because it sits, dissolves slowly, and delivers a localized blast of chlorine and acidity. The downside: on colored plaster, Pebble Tec, fiberglass, or vinyl, trichlor can burn or stain. If you use it, broadcast lightly after brushing the heads off, avoid piles, and keep pumps and cleaners off for a short window so granules can work without clumping.

Calcium hypochlorite offers a safer route on Pebble Tec and many plaster pools, particularly in cooler water where it dissolves more slowly. Broadcasting 6 to 7 pounds strategically over cleaned spots lets active chlorine sit on target longer, punching through the biofilm remnant. Some pros even break cal hypo tablets into chunks to mimic a slow‑release granular that stays put without the staining risk associated with trichlor. Skip this on vinyl, where any resting oxidizer can scar the liner. Dichlor dissolves too fast to be helpful here. After contact treatments, a whole‑pool shock to 30–50 ppm free chlorine can strip remaining colonies from walls and help the system reset, especially when combined with spot work on stubborn areas.

Severe cases call for heavier lifts. If black algae blankets walls and floors, a full drain and chlorine wash may be the only way to regain control. Acid washing rarely helps this organism and can deepen surface damage, making future infestations easier. Chlorine washing oxidizes the biofilm and cells without the collateral etching. For commercial pools, draining can also solve sideline issues like cyanuric acid overload and allows a faster, more controlled turnaround. Always protect yourself with a waiver, explain surface risks clearly, and plan a rapid refill and rebalance to avoid plaster stress.

Long‑term prevention is simpler than the cure but requires discipline. Keep free chlorine from dropping to zero—ever. Holding 5 ppm or more during recovery makes it hard for any survivors to regroup. Pair that with a steady copper level from a chelated source to keep ions active without staining or precipitation. Watch for early signs: pencil‑eraser dots on steps, tight clusters near returns, or new spots emerging on worn patches of plaster. Tackle them immediately with brush, targeted oxidizer, and filtration. If your route includes multiple pools, isolate tools and rinse gear between stops. With consistent chlorine, measured copper, and smart surface‑specific tactics, you can shut down the “pool plague,” protect surfaces, and keep water looking clean through peak season.