Black Algae

What is black algae?

Those dark specks are not true algae. They belong to a group of bacteria that grows as tiny dark spots and shields itself with a layer that resists chlorine. The pigment inside is the same one plants carry, but extra compounds sit on top of it and hide the green, so the dots look black.

The spots take hold in cracks and rough spots on pool surfaces, plaster especially. Green and yellow algae can appear in a pool on their own, but black algae always arrives from another place. It can hitch a ride on swimsuits, pool toys or scrub brushes after a visit to a lake, pond, river or some other infected pool.

Why is it so hard to kill?

Every cell builds a coating around itself, and that coating leaves black algae partly protected at normal chlorine levels. On its own, chlorine often cannot touch the living cells hiding below.

Brushing cracks that coating open. Once it is open, chlorine or algicide can get inside. An algicide is a chemical designed to kill algae.

How the treatment works

Three things have to work together here: hard brushing, a very high chlorine level and a filter that never stops. The chlorine comes from shock, which is one big dose of chlorine used to knock out algae and haze. The usual powder form goes by the name cal-hypo.

Free chlorine is the chlorine that is still free to sanitize the water, and it is the figure your test kit reports. Ppm is how it is measured, short for parts per million. With black algae you want free chlorine sitting at 10 to 20 ppm, a level far past what is safe for swimming.

On this site, black algae gets 4 lb of shock for every 10,000 gallons, the biggest dose we recommend. Powder can be swapped for liquid chlorine. They do the same job, so pick one and stick with that choice alone.

The pool-size box on the home page works out the amounts for your own pool.

Shock powder and liquid chlorine are sold in set bag and jug sizes. Get enough to cover the full amount your pool needs, pour only what you measured and keep the rest stored.

Cal-hypo puts calcium into the water. If calcium hardness (how much dissolved calcium the water holds) is already above 400 ppm on your test, go with liquid chlorine instead. Dark finishes call for extra caution, since granular chlorine may bleach out a dark finish, so liquid chlorine is the safer pick there.

While chlorine sits that high, keep pH (how acidic or basic the water is) in the range of 7.2 to 7.8, as the Illinois Department of Public Health recommends. For your safety, never mix pool chemicals, and wear protective gear when handling them. The CDC warns that mixing chemicals or handling them without protective gear can cause injuries.

Most strips max out near 10 ppm and cannot show whether you reached that goal. A drop kit that measures high free chlorine levels, usually sold under the name FAS-DPD, will give you the real number.

Swimming Again After Treatment

Wait to swim until you can see the main drain, the grate at the pool's deepest point, from the poolside. Free chlorine should sit between 1 and 4 ppm (or no lower than 2 ppm if the pool uses stabilizer), with pH kept in the range of 7.2 to 7.8. Those limits are set out by the CDC and the Model Aquatic Health Code.

Stopping Black Algae From Returning

Brushing 1 to 2 times a week helps keep black algae from coming back. It also pays to shock on a schedule and add a preventative algicide, beginning with one dose and following up each week.

Keeping black algae out matters just as much, because it cannot appear on its own. Wash swimsuits worn in lakes, ponds or rivers before they enter the pool. Consistent care and healthy chlorine levels do their part too, while too much stabilizer (cyanuric acid, which shields chlorine from sunlight) holds the chlorine back.