Black Algae in Your Pool in Los Angeles: How to Kill It

Black algae is not an alga in the usual sense. It is a colony of cyanobacteria, dark blue-green up close and black from the deck, and it grows as raised spots from pinhead to quarter size. Each spot builds layers over itself. The outer layer shields the living part from chlorine, and the part underneath grows into the pores of the plaster.

That is why a pool can get a full shock, go crystal clear, and still have the same spots on the steps a week later. The water was treated. The spots were not.

Is it black algae or a stain?

Dark spots on plaster have more than one cause, and two of them are treated in opposite ways. Before buying anything, spend five minutes on these checks.

• Run a finger over it. Black algae is raised and a little slimy or rough. A metal stain is flat and smooth, part of the surface.
• Brush it hard. Algae leaves a dark smudge in the water and the spot looks lighter for an hour. A stain does not move at all.
• Hold a vitamin C tablet on it for a minute. If the spot fades, it is iron or copper, not algae. Metal needs a stain treatment, and chlorine can make it darker.
• Look where it is. Algae picks shade and still water: steps, corners, behind the ladder, under the light niche. Metal stains tend to sit near a return jet or a corroding fitting, or wherever a dose of chemical landed.

Why black algae shows up in LA pools

It needs three things to take hold: a rough surface, a spot the chlorine barely reaches, and a stretch of weak chlorine. Plaster, pebble and quartz finishes give it the surface. Vinyl and fiberglass are smooth enough that it rarely roots there.

• Shade. A pool under oaks or sycamores in La Cañada or Altadena has corners that never get sun and barely get circulation. Those are the first to spot.
• Stabilizer out of range. When cyanuric acid creeps past 80 or 90, the chlorine reading looks fine but works slowly, and black algae gets its opening. The stabilizer guide explains how that happens with tablets.
• Short pump runs. A pump that runs four hours a day in August leaves the far steps with almost no fresh chlorine for twenty hours.
• Gear from a lake. Swimsuits, floats and wetsuits used at Castaic or Pyramid Lake can bring the bacteria home. Wash them before they go in the pool.

How to get rid of black algae, in order

Plan on a week of work for a few dozen spots, and two for a pool where they cover the steps and the deep end. Keep everyone out of the water from the first step until chlorine is back to normal.

1. Clean the filter first. A clogged filter slows circulation, and circulation is what carries chlorine to the spots. Hose a cartridge, backwash sand or DE.
2. Bring pH down to 7.2. Chlorine kills much faster at 7.2 than at 7.8. LA water drifts up, so this step usually needs acid.
3. Scrub every spot with a stainless steel brush. This is the step that decides the outcome. The steel bristles break the coating so chlorine can reach the colony. Use steel on plaster and pebble only; on vinyl, fiberglass or painted pools use a stiff nylon brush.
4. Rub the stubborn spots with a chlorine tablet. Wear gloves, break a trichlor tablet in half and press the broken edge onto each spot for a few seconds. Plaster pools only, because it bleaches liners and paint.
5. Shock at dusk and hold it there. Raise free chlorine to about 40% of your stabilizer reading, so 16 ppm at 40 CYA, and keep it at that level for several days. One big dose that fades by morning does little.
6. Brush the spots every day. Once or twice a day while chlorine is high. Each pass strips another layer.
7. Add a black algae algaecide if spots survive the first days. Most are copper-based. Copper can leave blue-green stains on plaster, especially if pH climbs, so dose by the label and not more.
8. Clean the filter again, and vacuum to waste if you can. Dead algae collects in the filter. Left there, it seeds the next outbreak.

Keep brushing for a week after the last spot disappears. The part in the plaster pores outlives the part you can see, and a spot that comes back in the same place is a spot that was never fully killed.

Will shocking the pool get rid of black algae?

Not by itself. Shock without brushing clears the water and bleaches the top of each spot, which makes it look beaten. The colony under the coating survives and is back in two or three weeks. Shock is half the treatment; the brush is the other half.

Green algae is a different case. It floats in the water, so shock and a good filter deal with it, and the green pool clean-up guide covers doses by water colour. If your pool is green and spotted at the same time, clear the green first, then work the spots.

When the spots keep coming back

Two or three rounds of brushing and holding chlorine, and the same spots return? Then the colony lives deeper in the plaster than a brush reaches. Old, rough, etched plaster is where this happens most.

The next step is draining the pool and washing the plaster with acid. An acid wash removes a thin layer of the surface, and the roots go with it. It is also the answer for dark shadows algae leaves behind after it dies. Draining is not a treatment on its own, though: an empty pool with live spots will have them again after the refill. If the plaster is worn through in places, an acid wash only buys a year or two, and resurfacing is the real fix.

Keeping it from coming back

• Brush the shaded steps and corners every week, including the spots behind the ladder and around the light. Algae that is disturbed weekly never builds its coating.
• Keep chlorine above zero between visits. A day or two at zero in summer is enough for spores to settle in.
• Check stabilizer monthly, and stop the tablet floater if it is past 80.
• Run the pump 8–10 hours a day in summer, with a return jet pointed at the corner that spotted before.

What it costs to have it treated

On weekly service, brushing and chlorine are part of the visit, and the monthly price starts at $159. An outbreak that is already established is quoted separately, like other heavy algae work, because it takes extra trips during the week to brush and redose. Algae recovery without draining runs $349–$749 for a residential pool, and an acid wash starts at $749. Both are on the price list.

Not sure if you are looking at algae or a stain? Send a close photo of the spots with a coin next to one for size, or call (818) 740-6099.