Why black algae resists chlorine
Black algae is not really an algae you wipe away. It anchors into the pool surface and forms a waxy protective coating that normal chlorine levels cannot penetrate. That is why those black spots survive a regular shock and come right back; the chlorine never reaches the living layer underneath.
So the whole strategy is about getting through that coating. Brushing is not optional here, it is the main event.
Brush hard to break the cap
Scrub each black spot aggressively with a stiff pool brush, a stainless steel algae brush on plaster, to break open the protective layer. This is physical work and you have to be thorough; any spot you miss will regrow. For stubborn heads, some people rub a chlorine tablet directly on the spot after brushing to drive chlorine into it.
Brush before you shock and again during the process. The point is to keep the surface broken open so chlorine can reach the roots.
Shock high and hold it
With the spots brushed open, raise free chlorine to the shock level for your cyanuric acid and keep it there. The shock calculator sets the target. Black algae needs that high level sustained, with repeated brushing, over several days, not a single dose. Run the pump continuously and brush daily.
Make sure your chemistry supports the chlorine working: pH in range so the chlorine is effective, which the pH calculator handles. Keep going until the spots are gone and do not return.
Clean everything and prevent a comeback
Black algae spreads from spores, so clean anything that went in the water: brushes, vacuum heads, toys, and swimsuits. Check and clean the filter, since it traps debris during treatment.
Prevention is the same as for any algae plus attention to circulation: keep chlorine in range every day, brush shady and rough spots regularly, and fix dead spots where water does not move. Black algae loves places the chlorine and the current do not reach.
Frequently asked questions
How do I get rid of black algae in my pool?
Brush each spot aggressively to break its protective coating, then raise chlorine to the shock level for your CYA and hold it there for several days while brushing daily and running the pump. Black algae roots into the surface and resists chlorine, so the mechanical brushing is essential, not optional.
Why does black algae keep coming back?
Because it roots into the plaster or grout and grows a protective layer that chlorine cannot penetrate. If you do not break that layer by brushing and kill the roots with sustained high chlorine, the visible spot returns from what survived underneath. Missed spots and dead-flow areas are common regrowth points.
Will shocking kill black algae?
Shocking is necessary but not sufficient on its own. You must first brush each spot hard to break its protective coating so the chlorine can reach the living algae, then hold shock-level chlorine for several days. Shock without thorough brushing usually fails against black algae.
Is black algae dangerous?
It is not directly harmful to swimmers the way bacteria are, but it signals that your sanitation and circulation have gaps, and it damages and stains plaster over time. It is also very hard to remove once established, so it is worth treating promptly and preventing with steady chlorine.
How long does it take to kill black algae?
Often several days to a week of sustained shock-level chlorine and daily brushing, because of how it protects itself. It takes longer than green algae. Keep the chlorine high and keep brushing until the spots are gone and do not reappear, then return chlorine to normal.