Black Algae: The Hardest Pool Problem, Treated Honestly
Updated September 2026
Quick answer: Black algae grows as dark dots with waxy protective layers and roots that penetrate plaster pores — chlorine can't reach it without mechanical assault. The working protocol: scrub each colony hard with a stainless brush (nylon for liners — though black algae on liners is rare), rub spots directly with a trichlor tablet, then hold elevated chlorine for days while brushing daily. Expect 2–4 weeks of repetition, not a weekend fix — and expect honesty: on heavily infested old plaster, full eradication sometimes needs acid washing.
Why shock alone never works
Black algae (actually cyanobacteria) builds layered colonies: a waxy cap that sheds chemicals, and root-like anchors sunk into plaster pores and grout lines. Free chlorine in the water only touches the cap — which is why a pool can hold 15 ppm for a week and the dots don't blink.
That biology dictates the treatment: every colony's cap must be physically broken so chlorine reaches the living tissue, and the roots must be hit repeatedly because any survivor regrows the colony in weeks. It's manual labor plus chemistry, in that order.
The protocol
One: scrub every visible dot with a stainless-steel brush (plaster/gunite only — use the stiffest nylon on other surfaces) until the shine is off the spot. Two: rub each spot directly with a trichlor tablet in a holder glove — placing near-pure chlorine on the wound. Three: raise pool chlorine to algae-kill levels and hold for several days, brushing all spots daily; run the pump continuously.
Repeat the scrub-and-tab cycle every few days. Colonies that stop re-darkening after brushing are dying; new satellite dots mean live roots remain. Typical full clearance runs 2–4 weeks of this rhythm — plan for the marathon and be pleasantly surprised by anything faster.
Aftercare and the honest escalation
After the last visible colony dies, keep chlorine at the top of range for two weeks, brush former spots every few days (roots die slower than caps), and chemically clean the filter. Check CYA — a long trichlor-heavy fight raises it meaningfully, and high CYA is how the next infestation gets its foothold.
The escalation nobody sells but everyone should know: on old, rough, heavily colonized plaster, roots survive in pores no brush reaches. If a properly executed month of treatment keeps failing, the surface itself is the problem — an acid wash (drain + professional) resets it, and a replaster is the terminal answer for crumbling surfaces. Most pools never get there; rough ones with years of neglect sometimes do.
FAQ
How do I get rid of black algae in my pool?+
Break each colony's protective cap with a stainless brush, rub spots directly with a trichlor tablet, then hold elevated chlorine for days while brushing daily — repeated over 2–4 weeks. Shock alone cannot penetrate the waxy layers.
Why is black algae so hard to kill?+
It's a cyanobacteria that grows waxy protective layers and sinks roots into plaster pores — water chemistry only touches the surface. Mechanical scrubbing plus direct chlorine contact is mandatory.
Does black algae mean I have to drain the pool?+
Rarely: the brush-tablet-elevated-chlorine protocol clears most cases without draining. Draining for an acid wash is the last resort for old rough plaster with deep-rooted, recurring colonies.