Problems & Fixes

How to Get Rid of Black Algae

How to identify and kill black algae: hard brushing, spot treatment, shocking to the right level, filter cleaning and how to stop the spots coming back.

By The Pooling Editorial TeamPublished 4 min read

Dark blue-green black algae spots on a white plaster pool wall under water, with a steel brush resting on the step
Illustration: The Pooling / AI-generated.

Key takeaways

  • Black algae forms stubborn blue-black spots, usually on plaster, and protects itself with a tough outer layer.
  • Break that layer by brushing hard every day, then hold chlorine at shock level until the spots are gone.
  • Prevent it with steady chlorine matched to your stabilizer, weekly brushing and good circulation.
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To get rid of black algae, you have to break through its protective layer and then keep chlorine high enough, for long enough, to kill it. In practice that means brushing the spots hard every day, spot-treating them, raising free chlorine to shock level for your stabilizer and holding it there until the spots vanish and the water passes an overnight chlorine test. It usually takes several days, sometimes a couple of weeks.

How to recognise black algae

Black algae is technically a type of cyanobacteria. It looks like:

  • Dark blue-green to black spots, from pinhead size to a few inches across
  • Raised, slightly slimy spots that don’t brush away easily
  • Clusters in shady areas, on steps, in corners and along grout lines

It grows mostly on plaster, pebble and tile grout, where rough surfaces let it anchor. It’s much rarer on vinyl and fiberglass. If spots on vinyl don’t brush off, they may be stains or metal deposits instead.

ProblemColorBrushes off?Typical location
Black algaeBlue-black spotsBarelyPlaster, grout, shady walls
Mustard algaeYellow or brownish dustEasily, then returnsShady walls and floor
Green algaeGreen tint or slimeYesEverywhere, often the whole pool
Metal stainsBrown, black or blueNoNear fittings or where metals entered

Why it’s so hard to kill

Black algae builds layers, with a tough outer coating that shields the cells underneath, and its roots grow into tiny pores in plaster. Chlorine that would kill ordinary algae in hours barely touches it unless you break the coating first. That’s why a single shock rarely works: the spots fade, then return. The treatment has to be physical as well as chemical, because neither chlorine nor algaecide reaches the roots until the coating is broken.

Step by step: killing black algae

  1. Test and balance the water. Get pH to about 7.2–7.4 so chlorine works at full strength, and know your stabilizer (CYA) level.
  2. Clean the filter. Backwash or clean it before you start; it will catch a lot of debris.
  3. Brush the spots hard. On plaster and pebble, use a stainless-steel brush to scrape the tops off every spot. On vinyl or fiberglass, use a nylon brush only.
  4. Spot-treat plaster. Many pool owners rub a trichlor tablet directly on each spot (wear gloves), or use an algae spot treatment. Never use tablets on vinyl, which they bleach.
  5. Shock to the right level. Raise free chlorine to shock level for your stabilizer. Our guide to how much shock your pool needs sets that level; with typical stabilizer, it’s well above normal chlorine.
  6. Hold it there. Test at least twice a day and add chlorine to keep it at shock level. Brush the spots every day.
  7. Run the pump continuously and clean the filter whenever pressure rises.
  8. Finish only when it’s clear: no visible spots, clear water, and free chlorine that drops by 1 ppm or less overnight.

Should you use algaecide?

Algaecide helps as a follow-up, not a replacement for chlorine and brushing. Copper-based algaecides work well against black algae but can stain plaster and turn blond hair green if overused, so follow the label closely. Polyquat algaecides are gentler and don’t stain. Never add algaecide at the same time as a heavy dose of chlorine unless the label says it’s compatible.

When nothing works

If spots keep returning after several full treatments:

  • Stabilizer may be too high, which makes chlorine sluggish. Our cyanuric acid guide explains how high CYA hides chlorine and how to fix it.
  • Plaster may be deeply pitted, giving algae permanent shelter. A pool professional can assess resurfacing or acid washing.
  • Circulation may have dead spots. Point returns toward problem corners.
  • Your test may be off. Old reagents can misread chlorine, so replace them each season.

How to prevent black algae

  • Keep free chlorine steady, matched to your stabilizer, and never let it hit zero.
  • Brush the whole pool weekly, especially shady walls and steps.
  • Keep pH in range so chlorine stays effective.
  • Run the pump long enough for good turnover, and clean the filter regularly.
  • Clean gear used in natural water before it goes in your pool.

If the whole pool has turned green, not just spotted, start with our step-by-step guide to fixing a green pool.

Frequently asked questions

Is black algae dangerous to swim in?

It isn’t usually considered a major health risk in itself, but it shows your chlorine isn’t keeping up, which means germs may not be controlled either. Stay out during heavy shocking and until chlorine returns to a normal level.

Can black algae grow on a vinyl pool?

It’s uncommon because vinyl is smooth. Dark spots on vinyl are more often stains, but if they are algae, use a nylon brush and chlorine, never a steel brush or tablets.

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

Several days for mild cases; two weeks or more for established growth. Keep brushing and holding shock level until the overnight chlorine test passes.

Sources

  1. CDC — Home pool and hot tub water treatment and testing
  2. Pool & Hot Tub Alliance

Every article is edited by a human and checked against our editorial policy. Spotted a mistake? Tell us.

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