What causes black algae in a pool?

Black algae (cyanobacteria) enters pools from contaminated water sources, soil tracked in on feet, and birds or other wildlife. It establishes itself on porous surfaces — plaster, concrete, and gunite — when chlorine levels drop below the threshold needed to prevent colonization. Once it forms a protective shell and roots its holdfasts into the surface, it's much harder to eliminate than it was to prevent.

Can you swim in a pool with black algae?

No. Black algae is cyanobacteria, not true algae, and some cyanobacteria strains produce toxins (cyanotoxins) that can cause skin irritation, respiratory issues, and gastrointestinal symptoms. The rough texture of the colonies also poses a skin abrasion risk. Stay out of the pool until the treatment is complete and all three exit criteria are met.

How long does it take to get rid of black algae in a pool?

Black algae in a pool typically takes 1–2 weeks of sustained shock-level FC combined with daily wire brushing to eliminate. Light cases with only a few small spots may clear faster. Deep infestations that have rooted into the plaster surface may require professional acid washing after the chemical treatment to remove remaining holdfasts.

Does black algae mean my pool is dirty?

Not necessarily in the way people assume. Black algae establishes in pools that have had FC drop below the CYA-based minimum — even briefly. It's more a sign of past chlorine lapses (or chronic low-level maintenance) than general dirtiness. Once established it's persistent regardless of current chemistry until you physically disrupt the protective shell and sustain high FC.

Will shocking a pool kill black algae?

Shocking alone will not kill black algae. The protective shell it forms blocks chlorine from reaching the cells beneath. You must wire brush each colony before and during every chlorine dose to physically break through the shell. Without brushing, even very high FC concentrations are blocked by the outer layer. Shock + wire brushing + sustained FC is the only chemical approach that works.

Part of: Pool Algae Series

Black algae in a pool: what it is and how to get rid of it

7 min read · August 5, 2026 · By Jeffrey Thompson

Black algae appears as dark blue-green or black spots on pool surfaces — small raised dots that don't brush off easily and keep coming back. It's actually cyanobacteria, not true algae, and it forms a hard protective shell that blocks chlorine from reaching the cells beneath. Elimination requires a stainless-steel wire brush to physically break that shell, combined with sustained shock-level FC held for one to two weeks.

Do not swim while treating black algae

Cyanobacteria can produce cyanotoxins that cause skin irritation, eye irritation, respiratory symptoms, and gastrointestinal illness. Stay out of the pool until the treatment is complete and all three exit criteria pass. The rough colony surface also poses a skin abrasion risk in the shallow end and on steps.

What black algae actually is

The name is misleading. "Black algae" is a common name for cyanobacteria — photosynthetic bacteria, not algae at all. Cyanobacteria are one of the oldest life forms on Earth and have evolved highly effective survival mechanisms, including the ability to form a hard, waxy outer shell (a polysaccharide capsule) that protects the colony from both physical disruption and chemical attack.

The biological structure has practical treatment implications:

The outer shell physically blocks chlorine from reaching active cells
The colony anchors to porous surfaces using root-like structures called holdfasts, which penetrate into plaster, concrete, and gunite
The spots you see are the tops of colonies; the holdfasts extend below the visible surface
Even after chemical treatment kills the surface cells, holdfasts can regenerate if they survive

Why you won't find it in vinyl pools

Black algae almost exclusively appears in plaster, concrete, and gunite pools. The holdfasts cannot penetrate smooth, non-porous surfaces like vinyl liner, fiberglass, or painted pool surfaces. If you have a vinyl liner pool and see dark spots, they are almost certainly staining (metal oxidation, tannins from organic debris) rather than black algae.

The rough, porous texture of plaster gives black algae the surface it needs to anchor. Plaster that has aged, roughened, or developed micro-cracks is particularly vulnerable because the holdfasts have more surface area to grip.

How to identify black algae vs. other dark spots

What you see	Most likely cause	Key tell
Dark blue-green spots, raised, rough to touch	Black algae (cyanobacteria)	Doesn't brush off; keeps coming back in same spots
Dark brownish spots, smooth, on floor	Iron or copper staining	No raised texture; responds to metal stain remover, not shock
Dark spots that look fuzzy, in corners	Dead organic debris or mold on plaster	Brushes off completely and doesn't return if FC is maintained
Large dark patches after algae treatment	Dead green algae remnants	Disappears over days with continued filtration and FC maintenance

The definitive test: try to brush a spot off with a stiff nylon brush. If it doesn't come off, or if it comes back within a few days in the exact same spot, it's black algae. If the spot brushes off cleanly and doesn't return, it's likely staining or debris.

Why regular shocking doesn't work

The most common black algae treatment failure is shocking the pool without wire brushing. Here's why that doesn't work:

Chlorine (hypochlorous acid) needs direct contact with the cells to kill them
The polysaccharide shell prevents contact — chlorine molecules cannot penetrate it effectively
The pool looks better after shocking because some surrounding free-floating cells are killed, but the anchored colonies with intact shells survive
Within days, the colonies regrow from surviving cells and the problem returns

Wire brushing physically breaks the outer shell before chlorine is added, exposing the interior cells. Without this mechanical disruption, even very high FC concentrations are blocked.

The full treatment process

Test and adjust pH to 7.4–7.6. High pH reduces chlorine's killing power. Fix it before starting treatment, not after. Use the muriatic acid calculator for dosing.
Wire brush every colony spot vigorously. Use a stainless-steel wire brush specifically designed for pool surfaces (not a nylon brush). Scrub each spot hard enough to visibly break the surface of the colony. If there are spots on the pool floor or walls, scrub each one individually. Do this immediately before each chlorine dose — the shell reforms over time.
If available, apply a trichlor tablet directly to each spot. Break a tablet into pieces and rub a piece against each colony for 30–60 seconds after wire brushing, before raising FC. The high local concentration of trichlor penetrates the freshly disrupted shell better than dilute pool water chlorine. This is an optional step but significantly improves outcomes.
Raise FC to shock level (CYA × 40%) with liquid chlorine. Pour chlorine around the pool with the pump running. Do this at dusk. See the shock calculator for the exact dose.
Add a quat or copper algaecide per the product label. Copper-based algaecides are effective against cyanobacteria but must be used carefully — never let FC drop below 1 ppm while copper is in the pool, or you risk permanent blue-green staining. Quat algaecides are safer for first-time black algae treatments.
Run the pump 24/7. Continuous circulation during the entire treatment period.
Retest FC every 4–6 hours and redose whenever it drops. Black algae consumes chlorine more slowly than green algae because the shell limits exposure, but FC still needs to be held at shock level continuously.
Wire brush every spot again the next morning before adding more chlorine. Repeat this brush-then-dose cycle every single day. Skipping even one day allows the shell to reform and blocks the day's chlorine dose.
Continue for at least 7–14 days. Even when the spots look gone, continue for 7 more days of shock-level FC and daily brushing. Surviving holdfasts will regrow if you stop too early.
Pass all three exit criteria before stopping. Water clear, CC ≤ 0.5 ppm, and overnight chlorine loss test passes (FC drops 1 ppm or less from dusk to dawn with pump off).

PoolChem Tracker logs your FC readings during multi-day treatments so you can see the trend and get notified when FC needs topping up. Track your treatment in the app or use the OCLT calculator to confirm when you're done.

What to expect during treatment

Days 1–3: Spots may look lighter after aggressive brushing but will still be visible. Continue. The daily brushing is disrupting the shell even if the visual change is subtle.
Days 4–7: Spots should visibly shrink or fade. If new spots appear, the original colony may have been larger than visible — continue treatment and brush the new spots.
Days 7–14: Most or all visible spots should be gone. Continue for at least a week after spots are no longer visible to kill remaining holdfasts.
After passing exit criteria: Return FC to normal maintenance level. The plaster surface may show discoloration or pitting where colonies were rooted. This is cosmetic damage from the holdfasts and the wire brushing; it does not indicate active algae.

If it comes back after treatment

If black algae reappears in the same spots within a few weeks of completing treatment, the holdfasts survived. This usually means:

The treatment period was too short — repeat the full process for a longer duration (14–21 days)
Wire brushing wasn't aggressive enough to penetrate to the holdfast depth — use a more aggressive wire brush or increase brushing pressure
The infestation has rooted deep enough that chemical treatment alone can't reach it

If repeated chemical treatment with daily wire brushing fails to clear the spots, the holdfasts may have penetrated too deeply for surface treatment. At that point, professional acid washing — which strips the outer layer of plaster — is the next step. This is a last resort; acid washing requires draining the pool and replastering within a few years.

Preventing black algae

Black algae establishes itself during windows when FC drops below the minimum for your CYA level. Prevention is the same as preventing any algae:

Maintain FC above CYA × 7.5% (non-SWG) or CYA × 5% (SWG) at all times — use the FC/CYA chart to know your target
Test FC 2–3 times per week minimum during swim season
Brush pool surfaces weekly, including the waterline and steps where cyanobacteria tend to first appear
Keep CYA in the 30–60 ppm range so that your minimum FC is achievable without excessive dosing

Track the FC trend that stops black algae before it starts

PoolChem Tracker logs every reading with a timestamp and shows the trend over time — so you can see when FC starts consistently dropping toward the algae threshold and act before colonies establish.

Pool Algae Series

How to kill pool algae (start here)
Pool turned green overnight — emergency triage
Mustard algae vs. green algae
Black algae in a pool
Why algae keeps coming back
Dead algae but water still cloudy