Black Algae in a Pool: Why It Returns and How to Kill It

What Is Black Algae in a Pool?

Black algae in pool plaster and grout is not true algae. It is a colony of cyanobacteria that anchors into the surface and hides under a protective slime layer. Normal chlorine only burns the top, so it returns unless the roots are broken.

It starts as a few dark dots, often in a corner, on a step, or along a grout line. They look like stains. You brush them and nothing happens. A week later there are more.

Black algae is the most stubborn growth a Central Florida pool can carry. Unlike green or cloudy pool water, it does not change the color of the water, so it is easy to ignore until it has spread across the plaster. The fix is not complicated, but it has to be done in the right order and finished completely, or it comes straight back.

How Do You Identify Black Algae in Pool Plaster?

Black algae has a look and a feel that separate it from dirt, stains, and metal deposits. Check for these five signs before treating anything.

• Raised spots. The dots sit slightly above the surface and feel rough or slimy under a hand. A stain is flat.
• Dark blue green to black color. Spots usually run from the size of a pencil eraser to the size of a coin, with defined edges.
• Shade and low flow. It favors corners, steps, benches, and the areas behind ladders and lights where circulation is weakest.
• Rough surfaces. Plaster, pebble finishes, and grout give it pores to anchor into. It is far less common on smooth glass tile and vinyl.
• It survives brushing. A nylon brush smears the top and leaves the spot in place.

The diagnosis matters because dark spots are not always algae. A metal stain from iron or copper does not respond to chlorine and needs a different treatment entirely. Shocking a pool for a stain wastes chemicals and a week.

Why Does Black Algae Resist Chlorine?

The name is misleading. Black algae is a cyanobacteria, the group the Environmental Protection Agency describes as blue green algae: organisms that make their own food from sunlight and live in fresh water, estuaries, and the ocean alike.

Pool colonies build a defense that ordinary green algae lacks. A 2025 study of black algae biofilms in swimming pools found them dominated by filamentous cyanobacteria wrapped in a dense layer of sugars and slime. That layer keeps chemicals from reaching the cells beneath it, and the organisms carry enzymes that defend against oxidizers like chlorine.

The same study found that standard algaecide doses hold back new growth, while mature colonies can need far higher concentrations. That matches what we see in the field. Chlorine at normal levels burns the exposed surface, the roots in the plaster survive, and the spots return within days.

What Causes Black Algae in Pool Water in Central Florida?

Black algae needs three things: a way in, a gap in the sanitizer, and a surface to hold. Central Florida supplies all three in late summer.

• A way in. Cyanobacteria live in lakes, springs, and the ocean. Swimsuits, floats, and toys that were in natural water carry them into the pool. So can debris blown in by a storm.
• Chlorine that dips. Heat and strong sun burn chlorine off quickly, and an afternoon storm dilutes what is left. The CDC recommends keeping free chlorine at 1 ppm or higher and pH between 7.0 and 7.8. Every hour below that line is an opening.
• Poor circulation. Short pump run times and dead spots behind steps and in corners let a colony settle without being disturbed.
• Aging plaster. Etched or pitted plaster gives the roots more to grip, which is why older finishes see it more often.

That is why late September matters. After a summer of heat, storms, and heavy swim traffic, many pools have run at the bottom of their chlorine range for weeks. Black algae takes hold in those gaps and is already rooted by the time the first spots are visible.

Holding steady numbers is the whole game. Our guide to pool water chemistry in Central Florida covers the ranges and the testing routine that keep them there.

How Do You Kill Black Algae in Pool Plaster and Grout?

Work in this order and do not stop early. Most black algae that returns was treated once, looked gone, and was left to regrow from the roots.

1. Test and balance the water first. Bring pH into range so the chlorine works at full strength.
2. Brush every spot hard with a stainless steel brush to break the protective layer. Use steel on plaster and pebble finishes only. Vinyl and fiberglass need a nylon brush.
3. On plaster, rub a chlorine tablet directly on each spot to push concentrated chlorine into the roots. Wear gloves.
4. Shock the pool to a high chlorine level in the evening, after the sun is off the water.
5. Add an algaecide labeled for black algae, dosed exactly as the label directs.
6. Brush the spots again every day for at least a week, and keep chlorine elevated through that week.
7. Run the filter around the clock, then clean or backwash it. The filter now holds dead organisms that can reseed the pool.

Expect the spots to fade over several days rather than vanish at once. If they remain after a full week of brushing and elevated chlorine, the colony sits deeper in the plaster than surface treatment can reach.

Clean the equipment too. Brushes, poles, vacuum heads, and floats that touched the algae can carry it back in. Our guide to pool filter and equipment care covers the filter side in detail.

Why Does Black Algae Keep Coming Back?

Black algae returns for one of four reasons, and each has a clear fix.

• The roots survived. Treatment stopped when the surface cleared. The fix is a full week of daily brushing and elevated chlorine.
• It was brought back in. Swimwear or gear from a lake, spring, or the beach carried it back. Rinse and wash anything that has been in natural water before it goes in the pool.
• The filter reseeded it. A filter left uncleaned after treatment releases it back into the water.
• Chlorine keeps dipping. If levels fall below range every few days, any surviving cells regrow. That is a routine problem, not a treatment problem.

How Do You Keep Black Algae From Returning?

Prevention is less work than any cure. The pools we care for that stay free of black algae share the same routine.

• Chlorine and pH tested and held in range, with more frequent checks during summer heat and after storms.
• Every wall, step, and corner brushed weekly, with extra attention to the low flow areas.
• Pump run times set for the season so the water turns over completely every day.
• The filter cleaned on a schedule, not only when the pressure gauge demands it.
• Swimwear and gear from natural water rinsed before it comes near the pool.

Weekly brushing is the step most owners drop first, and it is the one black algae exploits. A colony that is disturbed every week has a far harder time building the protective layer that makes it so hard to kill.

When Is Black Algae a Job for a Professional?

A few scattered spots caught early are a weekend job. It is time to bring in help when any of these are true.

• The spots cover large sections of the floor or walls.
• It returned after a full week of treatment.
• It has spread into the grout lines on tile or deep into a pebble finish.
• The plaster underneath is etched, pitted, or stained.

When black algae has rooted into aging plaster, surface treatment can only hold it back. The lasting answer at that point is the surface itself, and that assessment belongs with people who engineer and build pools.

This is the work our weekly pool maintenance service is built around. Every visit includes testing, balancing, and brushing every surface, so black algae never finds the gap it needs. For owners who want the whole property held to one standard, it is part of our luxury pool service.

Southern Pool Care is the care arm of Southern Pool Designs, building and caring for custom pools across Central Florida since 1997, with over $100M in completed work, an FSPA Design Award, and a 5.0 client rating. The people who set the care standard are the people who engineer the pools, which is why our water guidance reflects how these pools actually run. We work to the industry standards the Pool and Hot Tub Alliance sets.

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