Black algae is one of the most persistent and costly issues a commercial or public pool can face. For operators responsible for maintaining safe, code-compliant pools, the appearance of dark, stubborn, spot-like growths on plaster or concrete surfaces is a sign that water chemistry, sanitation, or maintenance routines need immediate attention.

Below, we break down what black algae is, why it develops in commercial and public pools, and the steps operators should take to remove it effectively and prevent future outbreaks.

What Is Black Algae?

Black algae in pools aren’t truly algae—they’re a type of cyanobacteria that share some properties with algae. However, they do not produce spores the way true algae do. Instead, they form tough, layered colonies with a protective biofilm that makes them difficult to remove once established.

These bacteria contain chlorophyll, so they are actually a blue-green color, but they also have a pigment that gives them a black color when combined with water (hence the name). They typically present as small (pin to quarter-sized) black or blue-black spots that don’t brush off easily.

Black algae is also an encapsulated bacteria: it has a hard outer shell that protects it and makes it uniquely challenging to kill. This upper layer shields the microorganism from chlorine and sanitizers. Its dense, interwoven layers enable it to survive even when chemical levels temporarily improve.

For pool operators, the biggest concern isn’t just appearance, it’s what this organism signals about pool conditions. Black cyanobacteria tend to establish themselves when water chemistry has fallen out of balance, circulation is insufficient, or brushing and surface maintenance have been inconsistent. In pools with high bather loads, where organic debris and sunscreen accumulate more quickly, black algae can find even more opportunities to take hold.

Why Black Algae Develops in Commercial and Public Pools

While black algae can technically appear in any body of water, it becomes especially problematic in commercial settings when consistent water management and mechanical circulation are disrupted. The most common causes include:

As you know, commercial pools must maintain stable, code-compliant chlorine or disinfectant levels at all times. Even short lapses, such as during peak bather loads, after heavy rains and winds, or when equipment malfunctions, create opportunities for black algae to seed and grow.

When pH, alkalinity, and stabilizer levels drift outside their recommended ranges, chlorine becomes less effective at sanitizing and killing microorganisms. This is one of the most common root issues operators encounter before black algae develops.

Black algae can enter the facility through:

• Swimsuits worn previously in natural bodies of water
  
  
• Pool equipment shared between facilities
  
  
• Inadequately cleaned vacuums, brushes, or maintenance tools

Because bacteria are microscopic, staff naturally don’t realize contamination has occurred.

Commercial pool surfaces—especially steps, corners, and coves—can develop “dead spots” where circulation is weaker. If these areas aren’t routinely brushed, microorganisms are more likely to colonize.

Undersized pumps, clogged filters, or incorrect turnover rates create conditions where water is not being circulated effectively. Black algae thrives in pockets of stagnant or low-flow water.

Operational Impacts of Black Algae

For public and commercial pools, black algae isn’t merely cosmetic. Its presence can indicate deeper sanitation issues that impact:

• Health: Black algae also harbors other bacteria like E. coli and its presence is a sign of poor quality water with low sanitation effectiveness, which can lead to serious health issues.
  
  
• Regulatory compliance
  
  
• Water clarity and perceived cleanliness
  
  
• Increased chlorine demand and chemical costs
  
  
• Potential guest complaints, loss of business, or closures
  
  
• Damage or staining to pool surfaces, leading to resurfacing expenses
  
  

The longer it remains, the more expensive it becomes to remove and the more likely it is to recur.

How to Remove Black Algae in a Commercial Pool

Eliminating black algae requires a coordinated approach across your pool’s mechanical systems, maintenance processes, and water chemistry. The following steps align with professional maintenance best practices.

Start by understanding your current state, then stabilizing pool water chemistry. Prioritize pH and Total Alkalinity (TA) at this point, ensuring TA levels are between 80-120ppm and pH between 7.2-7.4. 

Commercial pools often need hyperchlorination, or chlorine shock, to eliminate black algae.

Common shock guidance:

• Raise available chlorine significantly above standard operating levels
  
  
• Maintain circulation for 24 hours or longer
  
  
• Ensure pH is at the low end of the ideal range (7.2–7.4) prior to shocking to maximize chlorine efficiency. 

Always check and follow your local codes as well as the manufacturer guidance when applying a shock treatment.

Use a stainless-steel algae brush for plaster or concrete pools. While vinyl pools are less porous and less likely to deal with a black algae outbreak, if yours does, vinyl pools require a stiff nylon brush to prevent damage.

The goal is not just to loosen the algae. You also need to break through the protective layer so chemicals can penetrate. This step is essential and should be done aggressively across every visible colony.

General-purpose algaecides are not strong enough for cyanobacteria. Select a product explicitly formulated for black algae or blue-green algae.

Operators typically apply algaecide once chlorine levels begin to taper from the initial shock but remain elevated. This ensures deeper penetration into the bacterial colony.

For commercial pools, expect daily brushing for several days following treatment. Staff should document brushing as part of routine maintenance records for traceability and compliance.

Black algae can settle into filters and recirculate into the pool. Operators should:

• Backwash sand or DE filters
  
  
• Deep-clean or replace cartridges
  
  
• Inspect for clogs or worn media

A clean filter ensures chemical treatments can work effectively.

Once treatment is complete, maintaining tight chemical control is critical.

Recommended post-treatment ranges:

• Free chlorine: within code requirements for facility type
  
  
• pH: 7.2–7.6
  
  
• Total alkalinity: 80–120 ppm
  
  
• Cyanuric acid: per state/local code (typically 30–50 ppm for outdoor pools, 0 for indoor pools)

Commercial pools should monitor and record chemistry multiple times per day, particularly after an algae remediation event.

Preventing Black Algae Issues

Black algae can quickly turn into a costly, time-consuming headache for commercial and public pool operators. The good news? Most outbreaks are completely avoidable with consistent, well-documented water chemistry management. One of the easiest ways to stay ahead of issues like black algae is to use a reliable digital tool that standardizes testing, tracks trends, and flags imbalances before they become real operational problems.

That’s exactly what the PoolShark H2O app is built for. It simplifies accurate water testing, enables operators to record and store results in seconds, and helps you monitor chemistry over time—so early warning signs don’t slip through the cracks during busy days. By centralizing your logs and streamlining daily monitoring, PoolShark H2O helps facilities keep sanitizer levels steady, reduce chemical costs, and prevent expensive problems like black algae from ever taking hold.

A proactive approach, paired with the right technology, keeps your water safer, clearer, and fully compliant.

Black Algae in Commercial Swimming Pools FAQS

While black algae itself is not typically classified as a direct health hazard, it can cause health issues, can harbor actively harmful bacteria, and its presence indicates sanitation weaknesses, which can trigger inspection issues or corrective actions.

Yes. The microorganism can stain or pit plaster and concrete, increasing long-term resurfacing and maintenance costs.

Depending on severity, expect anywhere from several days to two weeks of brushing, hyperchlorination, and chemical monitoring.

Recurring outbreaks point to deeper systemic issues. These are usually inconsistent sanitizer levels, inadequate circulation, or unaddressed colony layers in the pool surface.