Dirt wipes off with a brush or your hand. Black algae won\u2019t. It appears as dark spots (usually blue-black or dark green) with a slightly raised, rough texture on plaster surfaces. If you scrub a spot hard and it comes back in the same place within a week or two, it\u2019s black algae. The spots often have a darker center with a lighter halo at the margin. One reliable field test: take a stiff pole brush to it. Dirt disappears. Black algae leaves a stain or comes right back.

Not on its own, and this is where most DIY efforts stall. Black algae (technically cyanobacteria) protects its cells behind a multi-layer barrier: an outer biofilm of proteins and polysaccharides, a peptidoglycan cell wall beneath that, and calcium carbonate deposits that form a mineral shield. A single chlorine shock raises free available chlorine temporarily, but it can\u2019t penetrate those layers deeply enough to reach the rhizoidal network embedded in the plaster. Shock without aggressive mechanical disruption (wire brushing to fracture the protective layers before chemical application) will clear the surface but leave the root system intact.

For an established infestation treated properly, expect a 14 to 21-day active treatment window. Day one is assessment and initial aggressive treatment. Days three through five are the sustained high-chlorine phase. Days seven to ten require a critical retreatment to catch cysts that began recovering. Days fourteen to seventeen are confirmation and final check. After that, prevention protocols start. The pool is usually swimmable at reduced capacity during treatment. We\u2019ll tell you specifically when your water is safe at each phase.

It can, but the reason is almost always the retreatment interval. Cyanobacteria cysts can survive in a dormant state inside plaster and begin recovering around day 10 to 14 after initial treatment. If the follow-up treatment doesn\u2019t happen in that window, the colony rebuilds. Most DIY failures, and many professional ones, happen because the retreatment is skipped or delayed. When we execute a full protocol with proper retreatment timing and the pool goes onto a regular maintenance schedule afterward, recurrence is uncommon.

Yes, in most cases. The majority of black algae infestations we see, including severe, long-established ones, respond to an in-water specialty treatment protocol. Draining is sometimes the right answer, but it\u2019s typically warranted when the plaster surface is too degraded to hold the treatment or when the infestation is so deeply embedded that the plaster itself needs replacement. We assess this on the first visit. Most pools we take on do not require a drain.

The initial visit is completely free. We come out, test the water, look at the plaster, diagnose the situation, and give you a flat quote. No assessment fee, no travel fee, no catch. Treatment typically runs $350 to $750 for the vast majority of pools. Your exact number comes from the free visit, not a phone estimate, and you\u2019ll have the flat quote in hand before any work begins. Occasional cases are outliers (larger pools, severely entrenched infestations, months-long battles) and those quote higher, but they\u2019re the exception, not the rule. Compare that to drain, acid wash, and replaster, which typically runs $6,000 to $10,000 in Dallas. When treatment is viable, it\u2019s typically one-third to one-fifth the cost of resurfacing.

Black algae itself isn\u2019t acutely toxic in a treated pool the way a cyanobacteria bloom in an open lake can be. However, the conditions that allow black algae to establish, low free chlorine and compromised water chemistry, do allow other pathogens to thrive. We recommend reducing swimming during active treatment phases when we\u2019re running elevated chlorine levels. The algae colonies are also physically rough and can cause skin abrasion, particularly for children. Once the pool tests clean and chlorine levels return to normal range, it\u2019s safe.

Mustard algae (yellow-green, powdery, wipes off easily, returns quickly) is true algae that responds well to standard shock and polyquat treatment. Black algae is cyanobacteria, a fundamentally different organism with structural defenses that true algae doesn\u2019t have. Mustard algae sits on surfaces; black algae roots into them. Mustard algae responds to a good superchlorination treatment over a few days. Black algae requires a multi-week protocol. Getting this identification wrong is why so many initial treatment attempts fail.

The recommendation is defensible and it\u2019s not laziness. After standard treatments fail, which they often do on established infestations, most training programs and manufacturer warranty paths point to drain, acid wash, and replaster as the documented resolution path. It\u2019s the one thing that removes both the plaster surface the algae has rooted into and the chemical environment supporting it. The problem is that it\u2019s frequently recommended as the first fallback when there are still specialty treatment options that haven\u2019t been tried. We pursue those options first.

The most common vector is contaminated swimwear, pool toys, or equipment that was in a body of water containing cyanobacteria: another pool, a lake, even a river. Black algae can hitch a ride on a swimsuit worn at a lake the week before and establish in your pool within days if your water chemistry creates a hospitable environment. In Dallas, storm debris is also a vector. Heavy rain events can introduce organic material and microorganisms from runoff. Once established, even a small colony can persist indefinitely with irregular maintenance.

For a mild, early-stage case (a few small spots on the plaster floor, caught quickly) a DIY approach with a wire brush, superchlorination, and strict retreatment on day 10 to 14 has a reasonable chance. For an established infestation, DIY success rates are low. The failure point is almost always one of three things: not brushing aggressively enough to fracture the cyst walls before chemical application, not sustaining high chlorine levels for long enough, or missing the retreatment window. If you\u2019ve tried twice and it\u2019s come back, call us.

We\u2019ll give you specific guidance at each phase. During the initial treatment phase we\u2019re running free available chlorine at 10 ppm or higher. Stay out until levels drop back to the 3\u20135 ppm range and pH is in balance. We test and confirm this before clearing the pool for use. For most protocols this happens within 24 to 48 hours after each treatment phase settles, with full clearance given after the final confirmation check around day 14 to 17.

Yes, over time. The rhizoidal network physically penetrates plaster by exploiting micro-pores and any existing surface irregularities. The colony also creates a localized acidic microenvironment as a byproduct of its metabolism, which can etch and soften plaster over time. The longer an infestation is left untreated, the deeper it roots and the harder it becomes to clear without also addressing plaster damage. In extreme long-standing cases, the damage to the plaster surface itself may require resurfacing regardless of treatment success.

The visual result at day 14 to 21 is a pool where the black spots are gone. Plaster may show faint staining where deep colonies were, which is normal and doesn\u2019t indicate active organisms. More importantly, water chemistry is stable without chlorine demand spikes during regular maintenance. We document this with photo reports taken at each phase. The real confirmation comes around day 30: if colonies haven\u2019t re-emerged with normal weekly service chemistry, the treatment worked.

Not through water, but yes through equipment and swimwear. If a tech uses the same brush or vacuum head in multiple pools without sanitizing between visits, they can transfer cyanobacteria. This is why we use dedicated equipment for black algae pools during treatment and sanitize everything before leaving. If you share pool toys or floats between pools, that\u2019s a transfer vector. Swimwear worn in an affected pool should be washed before use elsewhere.

The most common reason is incomplete treatment: cysts that survived the initial round and recovered during the retreatment window. The second most common reason is irregular maintenance afterward. Black algae does not re-establish in a well-maintained pool with consistent free chlorine and weekly brushing. In Dallas specifically, hard water and high summer evaporation can concentrate minerals and push water chemistry in directions that suppress chlorine efficacy. Pools that sit between service visits, especially during peak summer, are where recurrences happen.