Types of Pool Algae and How to Treat Each One

Green Algae: Treatment and Recurrence Prevention

Green algae is the most common type encountered in pool service and is typically the easiest to treat when caught early. It presents as green discoloration in the water, on pool surfaces, or on pool equipment, ranging from a slight tint in the water to a fully green pool. Green algae thrives when chlorine levels drop, either due to high chlorine demand from organic load, elevated cyanuric acid that renders chlorine less effective, or simply a missed service visit during hot weather. Treating green algae starts with testing the chemistry to understand the current state of the water. Check free chlorine, pH, alkalinity, and cyanuric acid before adding anything. If CYA is above 80 ppm, chlorine effectiveness is significantly compromised and the treatment dose will need to be substantially higher than in a pool with well-maintained stabilizer. Adjust pH to the 7.2 to 7.4 range before shocking, since low pH dramatically improves chlorine efficacy. Shock the pool with enough chlorine to reach 10 to 20 ppm free chlorine and sustain that level for at least 24 hours. Granular calcium hypochlorite shock or large doses of liquid chlorine are both appropriate, with the specific product choice depending on your preferred method and the pool's calcium level. Brush all affected surfaces thoroughly before and after adding shock to break up algae colonies and expose them to the sanitizer. Run the filter continuously and backwash or clean it after the initial treatment and again after the water begins to clear. A pool clarifier can help coagulate the dead algae particles for faster filter capture. Preventing recurrence requires identifying and addressing the root cause. If the algae developed because of low chlorine, investigate whether CYA is too high, whether the client's automatic chlorinator is undersized, or whether a service schedule change is needed. If phosphates are elevated, a treatment visit addresses the fuel source that made the outbreak possible in the first place.

Black Algae: Why It's Different and How to Treat It

Black algae is a fundamentally different problem from green algae and requires a completely different mindset in treatment. Black algae is a cyanobacterium that protects itself with a hard outer coating, which makes it extremely resistant to standard chlorination. It attaches to plaster, concrete, and grout surfaces with root-like structures that penetrate below the surface, making surface treatment alone insufficient. The result is an algae type that can survive aggressive shocking and reappear in the same spots repeatedly if not treated correctly. Black algae presents as dark blue-green to black spots or patches on pool surfaces, most commonly on plaster walls, steps, and in grout lines. The spots have a distinctive rough or slightly raised texture that distinguishes them from stains or dirt. When you brush a black algae spot, the outer coating brushes off temporarily and the spot may look lighter, but it regrows quickly if the underlying organism isn't killed. Treating black algae starts with aggressive physical action. Use a stainless steel brush, not a standard nylon brush, to vigorously scrub every black algae spot before adding chemicals. Breaking through the protective coating is essential for chlorine to reach the organism. After brushing, superchlorinate to the highest safe level the pool manufacturer allows, typically 20 to 30 ppm, and apply a concentrated dose of liquid chlorine or trichlor-based algaecide directly to each affected spot if your product labels permit spot treatment. Maintain elevated chlorine for 48 to 72 hours, brushing the spots daily. For persistent or severe black algae, a drain and acid wash is often the only complete solution, as the acid penetrates the plaster surface and kills the organism at root level. Be honest with clients that black algae is challenging and may require multiple treatment visits or ultimately an acid wash for full resolution. Document each treatment step so the client has a record of your efforts.

Mustard Algae: Identification, Treatment, and Prevention

Mustard algae, also called yellow algae, is often misidentified as dirt, sand, or pollen because of its pale yellow-green color and its tendency to settle on pool surfaces rather than turning the water green. It's more resistant to chlorine than green algae but less stubborn than black algae, and it has one characteristic that makes it particularly frustrating: it can survive on pool equipment, toys, swimsuits, and cleaning tools, reintroducing itself to treated pools from these contaminated surfaces. Mustard algae typically presents as yellowish patches or a dusty yellow film on pool walls, particularly in shaded areas. When you brush it, it disperses into the water as a cloud and then settles again within hours. This behavior is diagnostic: dirt or sand displaced by brushing stays suspended or sinks and doesn't actively reform on the surfaces. Treating mustard algae requires a dual approach targeting both the pool water and everything that's been in contact with it. Shock the pool aggressively and maintain elevated chlorine for 24 to 48 hours while brushing surfaces multiple times per day to keep the algae exposed to the sanitizer. Simultaneously, any equipment that's been in the pool, including the vacuum, brushes, and hoses, must be disinfected by soaking in a chlorine solution. Advise the client to wash and chlorine-soak their swimsuits and pool toys as well. Failing to disinfect these items is why mustard algae so often returns within a week of treatment. Prevention of mustard algae recurrence involves maintaining adequate free chlorine at all times, keeping phosphates low, and establishing a protocol for disinfecting pool equipment that's been moved between pools. If your technician uses the same brush or vacuum hose across multiple pools, there's a theoretical cross-contamination risk, though it's small for well-maintained pools with adequate chlorine. Dedicated equipment per pool is the safest approach for commercial accounts where water safety standards are higher.

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