How to Get Rid of Black Algae in Pool

Of every problem a pool owner can face, black algae is the one that strikes fear. It clings to walls and steps as dark, stubborn spots, roots deep into the surface, and shrugs off the chlorine levels that would kill most other algae overnight. Ignore it and it spreads, embedding itself further into the pool and becoming harder to remove with every passing week. The good news is that black algae can be beaten with the right method and a willingness to be aggressive. This guide explains what black algae is, why it is so persistent, exactly how to remove it step by step, and how to stop it ever coming back.

What Is Black Algae in a Pool?

Despite the name, black algae is not really algae at all. It is a form of cyanobacteria, also known as blue-green algae, one of the oldest and hardiest organisms on earth. That biology is exactly why it is so difficult to remove from a pool, and why it demands a more aggressive approach than green or mustard algae.

What It Looks Like

Black algae appears as small, dark spots or blotches, ranging from deep green to black, on the walls, floor, and steps of your pool. The spots are often slightly raised and slimy to the touch, with a protective surface layer that makes them feel different from a simple stain. Unlike a stain, they resist brushing and tend to return in the same spots if not fully eradicated.

Where It Hides

Black algae favours shaded, low-circulation areas where water sits still and sunlight is limited. Steps, corners, around ladders, and the shaded side of the pool are its usual homes. It takes hold most easily on rough, porous surfaces, which is why concrete and plaster pools are far more prone to it than smooth fibreglass.

Why It's So Hard to Kill

Two features make black algae so stubborn. First, it has a thick, protective outer layer that shields the living cells underneath from chlorine and other sanitisers. Second, it puts down roots that burrow into the pore structure of the pool surface, so even when the visible layer is killed, the roots survive and regrow. Beating it means physically breaking that protective layer by brushing, then driving sanitiser into the roots before they recover.

What Causes Black Algae?

Black algae does not appear out of nowhere. It takes hold when the conditions are right, and understanding those conditions is the first step to preventing it.

Poor Circulation and Dead Spots

When your pump or filter is not moving water efficiently, certain areas of the pool become dead spots where water stagnates and sanitiser barely reaches. These low-flow zones, often in corners and around steps, are exactly where black algae establishes itself.

Low Sanitiser Levels

Chlorine is your pool's primary defence. When levels drop too low, whether from heavy use, hot weather, or simple neglect, algae of every kind gets a foothold, and black algae is quick to exploit the gap.

Warm, Sunny Weather

Australia's climate is ideal for black algae. It thrives in warm water and bright sunlight, so the same conditions that make a pool inviting in summer also accelerate algae growth when sanitiser and circulation are not keeping pace.

Contamination From Outside

Black algae spores are often introduced from outside the pool. Swimwear, toys, inflatables, or cleaning tools that have been used in a natural body of water such as a lake or river can carry spores straight into your pool, where they settle into shaded, porous spots.

Is Black Algae in a Pool Dangerous?

Black algae is more than an eyesore. It signals a pool that is not properly balanced or circulated, and it brings real risks worth taking seriously.

Slippery Surfaces

Black algae forms a slimy layer on the surfaces it colonises, making steps, walls, and floors slippery underfoot. That raises the risk of slips and falls, particularly for children and older swimmers.

Skin, Eye, and Respiratory Irritation

As cyanobacteria, black algae can harbour other bacteria and, in some cases, release compounds that irritate the body. Swimming in an affected pool can contribute to skin rashes, eye irritation, and the kind of recreational water illnesses that come from poorly sanitised water. It is best to keep out of the pool until the algae is gone and the water is balanced.

Damage to Pool Surfaces

Left untreated, black algae roots etch into concrete and plaster, leaving staining and pitting that can outlast the algae itself. Severe, long-term infestations can damage the surface badly enough to require resurfacing, so prompt treatment protects the pool as well as the swimmers.

How to Get Rid of Black Algae in Your Pool Step by Step

Removing black algae takes persistence. Work through these steps in order, and be prepared to repeat them, because a single treatment rarely finishes the job.

Step 1: Test and Balance the Water

Start by testing your water and bringing it into range. Aim for alkalinity of 80 to 120 ppm, then lower your pH towards 7.0 to 7.2 for the treatment, because chlorine works far more effectively at the lower end of the range. Keeping a close eye on your water chemistry throughout the process is what makes the chemical side of the treatment work.

Step 2: Brush the Affected Areas Hard

This is the step most people underdo, and it is the most important. Use a stiff-bristled brush for plaster or a steel brush for concrete, and scrub the spots aggressively to break the protective layer covering the algae. Until that surface layer is broken, no chemical can reach the cells beneath. Brush every visible spot thoroughly.

Step 3: Shock the Pool Heavily

Once the spots are brushed open, shock the pool with a strong dose of chlorine, double or triple the normal amount for a stubborn infestation. Shock at night so the sun does not burn off the chlorine before it can work, and run the pump continuously to circulate it.

Step 4: Apply Algaecide and Black Spot Treatment

Follow the shock with an algaecide suited to black algae, and a dedicated black spot treatment if the infestation is severe. These reach into the roots and help prevent the colony re-establishing. Apply them to the brushed spots while the chlorine level is still elevated.

Step 5: Brush Again, Daily

Black algae is relentless. Brush the affected areas hard every day for several days, even once the spots appear to fade, because the roots are almost certainly still present below the surface. Consistent daily brushing is what eventually exhausts the colony.

Step 6: Vacuum and Clean the Filter

Vacuum any dead algae and debris to waste rather than back through the filter, so spores are not recirculated into the pool. Then clean the filter thoroughly: backwash a sand filter or soak a cartridge in filter cleaner, because black algae spores will happily hide in the filter and reinfect the water.

Step 7: Retest and Follow Up

Once the spots are gone, retest and rebalance the water, returning pH to the normal 7.4 to 7.6 range. Then shock again within a week to kill any spores that survived the first round. Black algae returns if the roots are not fully eradicated, so this follow-up is essential.

When to Consider an Acid Wash

For severe infestations that resist repeated treatment, an acid wash may be the only complete solution. The pool is drained and a diluted acid solution is applied to the surface, stripping away the thin outer layer of plaster where the algae has rooted. Because it removes part of the pool surface, an acid wash is a last resort, best carried out by a professional and never repeated often. For smaller, isolated patches, a spot treatment with diluted acid and a brush can target the area without draining the pool.

Black Algae vs Other Pool Algae

Not all algae is equal, and knowing which type you are dealing with tells you how hard you need to work. This is how black algae compares with the more common types:

If your problem is cloudy or green water rather than dark rooted spots, our guide on why your pool is green and how to fix it quickly and our guide on how to fix cloudy pool water will get you there faster.

How to Prevent Black Algae Coming Back

Beating black algae once is hard work. Keeping it away is far easier, and comes down to denying it the conditions it needs.

Keep the Water Balanced

Consistent water chemistry is your first line of defence. Keep chlorine between 1.5 and 3 ppm, pH at 7.4 to 7.6, and alkalinity at 80 to 120 ppm, and test at least weekly, more often in hot weather or after rain.

Brush and Shock Regularly

Brush the walls, steps, and shaded corners weekly even when the pool looks clean, and shock the pool weekly through summer. This disrupts spores before they can establish and catches anything your sanitiser has missed.

Improve Circulation

Run your pump and filter for 8 to 12 hours a day so no part of the pool becomes a dead spot. Pay attention to shaded corners and steps, and direct return jets to keep water moving in the areas black algae favours.

How Elite Pool Covers Can Help

Black algae thrives on sunlight, still water, and the spores and debris that drift into an open pool. A quality pool cover works against all three, which makes it one of the most effective long-term defences against black algae taking hold in the first place.

Blocking the Sunlight Algae Needs

Like all cyanobacteria, black algae depends on sunlight to grow. A solar pool cover blocks the light reaching the water when the pool is not in use, depriving algae of the energy it needs to establish and spread, while also cutting evaporation and helping hold your chlorine and pH steady.

Keeping Spores and Debris Out

Many black algae outbreaks start with spores carried in on the wind or in organic debris. A leaf and debris pool cover keeps leaves, dust, and contaminants out of the water, reducing both the spores that seed an infestation and the organic load that lets it feed. Our guide on why you need a pool cover explains the wider benefits.

Australian-Made Covers Built to Last

Elite Pool Covers has manufactured premium Australian-made covers since 1989, and was the first Australian company to design and build an automatic pool cover. Whether you need a solar cover, one of our pool cover rollers to make daily use effortless, or a fully automated pool cover, every product is engineered to withstand tough Australian conditions. Browse the full range of pool covers to find the right match for your pool.

Frequently Asked Questions