Black Algae vs. Green Algae: How to Tell the Difference and Treat Each

Not all pool algae is the same problem wearing a different color. Green algae is the common one: it clouds the whole pool and usually shows up after a few days of low chlorine. Black algae is rarer, more stubborn, and needs a different approach entirely because of how it protects itself. Treating black algae like green algae (or the other way around) is one of the most common reasons a treatment doesn't work. Here's how to tell them apart and what actually works for each.

How to tell them apart

The color is a starting clue, but texture and location matter more. Green algae tends to be free-floating or lightly attached, turning the whole pool cloudy or giving surfaces a slimy green film that wipes away easily with a finger. Black algae shows up as small, dark spots or bumps, usually blue-green to black, that feel gritty or raised when you run a hand over them, almost like a barnacle. It tends to cluster in shady, low-circulation spots: corners, steps, behind ladders, and along pool liner or plaster seams.

Why black algae is the harder problem

Black algae isn't just a color variant of green algae, it behaves differently in the water. It forms a protective outer layer and sends root-like filaments into small pores and cracks in plaster, grout, or rough surfaces. That outer layer blocks chlorine from reaching the organism underneath, which is why simply raising chlorine and waiting rarely clears it. The surface has to be physically broken open before chemistry can do its job.

Treating green algae

Green algae responds well to chlorine once you get ahead of it, since it's mostly suspended in the water column or loosely attached to surfaces.

• Brush all visible algae off walls, steps, and the floor to break up clumps and expose more surface area to chlorine.
• Test and correct pH and total alkalinity first (ideal pH 7.4-7.6, total alkalinity 80-120 ppm); chlorine works less effectively outside that range.
• Shock the pool using breakpoint chlorination: as a rule of thumb, dose to roughly 10x your combined chlorine reading, or a standard shock of about 10 ppm free chlorine if combined chlorine is low or unknown. These are estimates, so retest after dosing. Skimmer's chlorine calculator can work out the actual gallons of liquid chlorine or pounds of cal-hypo for your pool size.
• Run the filter continuously (24 hours a day) until the water clears, and brush again once or twice during that window.
• Backwash or clean the filter once the water clears, since a lot of dead algae ends up trapped there and can cloud the water again if left in place.

Most green algae blooms clear within 24 to 48 hours of consistent shock and filtration. If the water is still cloudy or green after that, retest and dose again rather than assuming the first shock failed. Chlorine demand from a heavy bloom can eat through a normal dose faster than expected.

Treating black algae

Black algae needs mechanical work before chemistry, not instead of it. Skipping the brushing step and just shocking harder is the most common reason black algae keeps coming back.

• Brush aggressively, directly on each spot, using a stainless steel brush on plaster/gunite or a stiff nylon brush on vinyl and fiberglass (steel bristles will scratch those surfaces). The goal is to physically break open the protective layer, not just knock loose surface debris.
• Brush every spot daily for several days in a row, even ones that look untouched by chlorine. This is the step most people cut short.
• Shock heavily after each brushing session, since the exposed algae underneath the broken layer is far more vulnerable to chlorine than the intact spot was.
• Consider an algaecide labeled for black algae or a longer soak with a concentrated chlorine dose directly on stubborn spots, in addition to (not instead of) daily brushing.
• Keep the filter running continuously and retest chlorine daily until the spots are gone and stay gone through a couple of normal chlorine cycles.

Preventing both from coming back

Algae of either color is almost always a symptom of a chlorine or circulation gap, not bad luck. The prevention list is nearly the same for both:

• Keep free chlorine consistently in range (1-3 ppm ideal, 1-4 ppm acceptable) rather than letting it swing low between tests.
• Run the pump long enough each day to fully turn over the water, especially in hot weather when algae grows fastest.
• Brush the pool weekly, even when it looks clean, paying extra attention to shaded corners and low-flow spots where black algae tends to start.
• Keep cyanuric acid in the 30-50 ppm ideal range so your chlorine isn't getting burned off by sunlight faster than it can work.
• Log your tests so you can catch a slow chlorine drift before it turns into a bloom. Skimmer tracks every reading against these ranges automatically, which makes a slipping trend obvious well before the water turns green.

Once you've had a black algae outbreak in a particular spot, keep an eye on that exact location going forward. Even after a full treatment, it's often the first place a new bloom starts if chlorine drifts low again, since some root structure can survive deep in a porous surface.