It starts as a few dark spots on the deep end wall, about the size of a pencil eraser, in the corner that never gets much sun. They get brushed, they get shocked, they fade. Three weeks later they’re back in the same spots, plus a couple of new ones. If the pool has gone through that cycle more than once, the question isn’t how to kill it. It’s whether the plaster is the problem, which is where our pool draining and acid washing work sometimes comes in.

Black algae is stubborn in a way green algae isn’t. Green algae floats in the water and loses to chlorine. Black algae grows into the surface and waits.

What black algae in a pool actually is

It’s not really algae. BioLab’s EPA-registered label for its Terminator copper algaecide lists it as “blue-green (frequently called black or spot algae),” which is the old name for cyanobacteria.

That matters because it behaves differently. Green algae spreads through the water. Black algae stays put, builds a slick outer layer, and sends growth down into the surface it’s sitting on. The dark spot you see is the top of it.

Our post on green pool algae removal covers the regular kind, where the whole pool turns and the fix is chemistry plus filtering. This is a different fight.

Why it keeps coming back

Three reasons, usually all at once.

The roots survive. Brushing knocks off the visible head. Shock kills what it touches. But the part growing down into the plaster pores is protected, and it regrows the same spot. That’s why it reappears in exactly the same places.

The surface gives it somewhere to live. Smooth new plaster is hard for it to grip. Older plaster that’s rough, pitted or etched is full of tiny pockets. Grout lines and the joints around steps and lights do the same thing.

The spot it picked has bad circulation. Shaded corners, behind ladders, the backs of steps, and anywhere water doesn’t move much. Those spots run lower chlorine than the rest of the pool even when the test kit at the skimmer says everything’s fine.

Brushing: the part that does most of the work

Most black algae pool treatment that fails, fails right here. Both algaecide labels we checked start with the brush. Applied Biochemists’ Black Algaetrine label says to “brush algae spots prior to application,” then, if needed, once a day for two days after. BioLab’s label says to brush thoroughly before treatment and again the next day.

The point isn’t to scrub the spot away. It’s to tear open that slick top layer so chemicals can get into the colony instead of rolling off it.

On plaster, that means a stainless steel brush, worked hard on each spot. It’s a different tool from the nylon brush that comes with the pool. A stainless brush will scratch vinyl, fiberglass and some tile, so match the brush to the surface, which is what BioLab’s label means by a “surface-compatible brush.”

Spot treatment that actually reaches it

Once the spot is scrubbed open, the goal is to put a lot of sanitizer right on it, not spread it thin across the whole pool.

The copper algaecide route is a labeled method. BioLab’s directions are to adjust pH to 7.2 to 7.6, apply 16 fluid ounces per 10,000 gallons “in areas of heaviest algae growth” with the pump running, circulate for 24 hours, brush again the next day, and repeat after 2 to 4 days if the algae persists. Applied Biochemists’ product calls for 12 ounces per 10,000 gallons for visible algae.

The chlorine route is just as common: a chlorine tablet held against the scrubbed spot, or granular chlorine sprinkled onto it with the pump off so it sits where you put it. It works. Just don’t leave a tablet parked on plaster for hours, because that concentrated a dose bleaches and etches the surface.

One caution on copper products: copper is one of the metals behind blue-green stains on plaster, and the label for the Nature2 mineral system even tells owners not to combine it with copper-based algaecides. Dose by the label, not by feel.

Keeping chlorine up while it dies

This is where most comebacks happen. The spots fade, everybody relaxes, chlorine drifts low over a hot week, and whatever survived in the plaster grows back.

CDC’s home pool water treatment guidance puts the floor at 1 ppm free chlorine in a pool without stabilizer and 2 ppm with cyanuric acid. A pool fighting black algae should not be anywhere near that floor. It needs chlorine held steadily above it for weeks, not a big shock followed by a slow slide to zero.

Circulation matters too. If the spots live in a dead corner, aim the returns to push water there, and make sure the pump is actually running long enough. Our post on how many hours to run a pool pump covers the math.

Black algae vs a stain: check before you fight it

Not every dark spot is algae. Metal stains and organic stains can look nearly the same from the deck.

If it’s flat and won’t budge under a brush, you’re probably looking at a stain, and that’s a different treatment altogether.

When black algae means a drain and acid wash

Sometimes the honest answer is that the surface is the problem. Signs it’s gone past spot treatment:

The spots come back after two or three full rounds of brushing, dosing and holding chlorine. At that point the colony is living deeper than you can reach with water in the pool.

The plaster is rough. If your palm snags on the shallow end floor, the whole surface is a home for this stuff.

It’s spread across a lot of the surface, not just a few spots in one corner.

An acid wash takes off a thin top layer of plaster, which is where black algae anchors, and it gives you a clean, smoother surface to start over on. Our post on what a pool acid wash actually does covers the limits, mainly that a surface only has one or two washes in it. If the plaster has already used those up, or it’s pitted all the way through, the lasting fix is a resurface. Our post on how many years plaster lasts is the next read.

Keeping it from coming back

The prevention list is short and boring, which is probably why it gets skipped. Chlorine that never bottoms out. A brush on the walls and steps every week, especially the shaded parts. Returns aimed so the corners actually get water. A filter that’s clean enough to move the flow the pump is rated for.

That weekly brushing is a big part of what our regular pool maintenance is for. Black algae that gets a brush every week rarely gets established. Black algae that gets ignored for a month in a shady Thousand Oaks backyard under an oak is a different story.

Asked at the pad, every week

Can I just shock the pool? Shock alone almost never finishes black algae. The slick top layer and the part rooted in the plaster both survive it. Brush first, then treat.

Is black algae dangerous to swim in? The bigger concern is what it says about your chlorine. A pool where it’s growing has spots that aren’t being sanitized well, and that’s worth fixing before anyone swims.

Why is it only in one corner? That corner probably has the worst circulation or the most shade, and maybe the roughest plaster. Black algae picks the weakest spot in the pool.

Will a new pool finish stop it? A smooth new surface makes it much harder for black algae to anchor. It won’t stop it if the chlorine and circulation problems that let it in are still there.

Tell us how long it’s been fighting back

Send a photo of the spots and a rough history (how many times it’s been treated, how fast it returns) through our contact page. That usually tells us whether it’s a brushing problem, a circulation problem or a plaster problem.

Or call (805) 586-2055. We do the weekly brushing, the spot treatments, and the drains and acid washes if it gets that far.