Black algae in pool: identify it, then get rid of it

What black algae actually is

Black algae is a cyanobacterium: a photosynthetic organism that forms a protective waxy coating and anchors into porous or rough surfaces. Underneath that coating it is really dark blue-green, which is why a fresh smear looks almost black on a wall and green on a white cloth.

Because it grows into the surface rather than sitting on top of it, the removal problem is mechanical first and chemical second. Anything that only adds chemicals and never brushes will lose.

Black algae or a black stain?

This is the first thing to settle, because metal staining is a different problem with a different fix — and treating a stain as algae wastes weeks.

If it smears, treat it as algae. If it does not, it is a stain and you should not be adding algaecide to it.

Where it hides

• Plaster walls and steps, especially where the surface is rough or already pitted.
• Tile grout lines, which give it something to grip.
• Around return fittings, the skimmer throat and light niches.
• Corners and anywhere circulation is weak.
• Under removable ladders and rails that sit in the water.

Every one of those places has to be brushed. A wall-only pass leaves the organism exactly where it was.

The non-chemical half: brush it, then keep brushing

• Use a stainless steel brush on plaster. On a vinyl liner or a fibreglass shell, use nylon — a stainless brush will scratch the surface permanently.
• Brush daily while it is active. You are physically breaking the waxy layer so sanitiser can reach the organism.
• Keep the filter clean. Dead algae becomes filter load, and pressure tells you when to clean.
• Improve circulation toward the affected areas — aim a return at a stubborn patch if you can.
• Remove and scrub anything that sits in the water: ladders, rails, toys, the vacuum head.

The chemical half — and why we do not give you a number

Eliminating black algae means holding chlorine at an elevated level, matched to your cyanuric acid level, for long enough that the organism is killed rather than knocked back. That is a dose, and the right value depends on your stabiliser reading — which is exactly the kind of calculation we have not published yet.

Our rule is simple: we publish dose amounts only after a credentialed pool professional has reviewed them. So this page gives you the identification and the mechanical method, and stops there.

Why it keeps coming back

• Cyanuric acid is too high. Stabiliser protects chlorine from sunlight but also slows its oxidising power, so the same free chlorine reading does less work. See the CYA calculator — this is the most common hidden cause.
• Free chlorine is allowed to hit zero, even briefly. Algae does not need a long window, it needs one.
• Circulation is poor in the affected area, so sanitiser never reaches it.
• It was never brushed — only shocked.
• The filter is holding the organism and reseeding the pool.

Prevention

• Keep free chlorine from bottoming out, rather than reacting after the fact.
• Keep CYA in range — overshooting stabiliser quietly weakens everything you add.
• Brush the surfaces regularly, especially rough plaster and grout.
• Clean the filter on pressure, not on a schedule.
• Follow the maintenance schedule so problems are caught while they are still small.

Black algae questions