Black Algae in a Pool: How to Identify and Treat It

Black algae is not really algae. It is a cyanobacterium (a type of bacteria), and that explains why it is so stubborn. This page shows how to tell it from stains, how to treat each surface, and when to call a pro.

Black algae or a stain?

Dark spots on a pool wall are often something else. Before you treat for algae, run these quick checks on one spot:

Spot test: black algae, metal stain, or organic stain
Check	Black algae	Metal stain	Organic stain (leaves, berries)
Look	Raised, round, blue-black or green-black dots, often with a defined edge	Flat, brown, gray, or black; can be blotchy or follow a pattern near fittings	Flat, brown or tan, often the shape of what sat there
Stiff brush	Top layer scuffs a little; the spot stays	Nothing comes off	Nothing, or it lightens slowly
Vitamin C tablet rubbed on the spot	No change	Often lightens or lifts	Little change
Where	Plaster, grout lines, rough or shaded areas, steps	Near returns, heater outlets, or where metal chemicals were added	Floor, under trees

A spot that lifts with vitamin C is metal, not algae. Treat it as described on pool stains; extra chlorine won't remove it. Not sure what color you're looking at? The pool algae types guide compares all of them.

Yellow-brown dust that brushes off easily is more likely mustard algae; a slick pink film on fittings is pink slime, a bacterium.

Why black algae is so hard to kill

Orenda, a pool chemical maker, describes the outer layer of black algae colonies as highly resistant to chlorine. Under it, the colony sends anchors into the small pores of plaster and grout. Chlorine in the water reaches the surface of the spot, but not much deeper. Two things follow:

Brushing is not optional. You have to break the protective layer so chlorine can reach the cells inside.
Spots come back from the roots. If you stop too early, they regrow in the same places.

Rough, porous surfaces give it the most grip: plaster, exposed aggregate, and grout. Smooth vinyl and fiberglass give it little to hold on to, which is why it is much less common there.

Treatment step by step

Test free chlorine (FC), pH, and stabilizer (CYA). Bring pH into range first, using the pH calculator if it is high, because chlorine works better at lower pH.
Brush every spot hard with the brush that suits your surface (table below), until the dark top layer is scuffed. Do this before you add chlorine and again each day.
Raise chlorine with a product labeled for algae control in pools, using the amount its label gives for algae or shock treatment. Scale it to your gallons. Don't go above the amount on the label, and keep everyone out of the water until FC is back at or below the label's swim limit.
Run the pump around the clock during treatment and clean the filter when the pressure rises. Brushed-off material ends up there.
Retest FC every morning and evening. Keep chlorine at the level the label directs for treatment and keep brushing the spots daily.
When the spots are gone, keep brushing those areas a few times a week for several weeks and check for regrowth in the same places.

An algaecide can help as a second step on spots you have already brushed. Use one whose label lists black algae, and dose it by that label. The algaecide calculator scales a label dose to your pool. Algaecide is a helper, not a replacement for brushing and chlorine.

How high chlorine goes during treatment, and for how long, is set by your product's label, not by us. Some water-care methods use levels above the swim limit on many labels; we don't give those numbers here. For shocking in general, see how to shock a pool.

Brush and method by surface

What to use on each pool surface
Surface	Brush	Notes
Plaster, concrete, gunite	Stainless steel brush on each spot; nylon for the rest of the wall	Most common place for black algae. Taylor's kit card recommends stainless steel on concrete.
Pebble or exposed aggregate	Stainless steel or stiff combination brush on spots	Rough texture hides colonies. Follow the finish maker's care guide.
Tile grout	Small stiff brush or a grout brush	Grout lines are a common spot. Brush along each line.
Vinyl liner	Soft nylon only	Never use a steel brush; it scratches and can tear the liner. On vinyl, small black spots tend to cluster at seams, steps, ladder rails, and light rings.
Fiberglass and painted pools	Soft nylon only	Steel brushes scratch gel coat and paint. Rare on smooth fiberglass; check the spot test first.

On vinyl, dark blotches that keep coming back in irregular shapes and don't respond to brushing may be growing on the back of the liner, between the liner and the wall. A trade article for pool pros notes that these stains often return after treatment. That is a job for a liner professional, not more chlorine.

Black algae in a salt water pool

A salt pool is a chlorine pool: the salt cell makes chlorine from salt, so black algae is treated the same way. Two differences:

A salt cell adds chlorine slowly. Raising the output to the maximum usually won't reach a treatment level quickly. Many owners add a chlorine product labeled for shocking pools and follow its label. Check your cell's manual for what it says about shocking.
Salt pools usually run higher stabilizer (CYA), often 60–80 ppm. Higher CYA means more chlorine is needed to do the same work, so check CYA before you start.

More on running a salt pool: salt water pool care.

When to call a pro

The spots come back in the same places after several weeks of brushing and label-directed treatment. The colony may have reached deep into old, rough plaster.
Spots cover large areas of the floor and walls.
You have a vinyl liner and the spots don't match the spot test.

A pool professional can confirm what it is and talk through options, including acid washing or resurfacing for badly affected plaster.

Keep it from coming back

Keep FC at or above the minimum for your CYA every day. The pool chlorine levels page has the full table.
Brush the whole pool weekly, including steps, corners, and behind the ladder.
Run the pump long enough to pass all the water through the filter each day, and keep the filter clean.

The calculator below shows the minimum and target FC for your stabilizer. With its default CYA of 40 ppm, the minimum is 3 ppm and the target is 3–5 ppm. For shocking, it points you to your product's label.

Safety. Always follow the product label. It is a violation of federal law to use a pesticide product in a manner inconsistent with its labeling. Never mix chlorine products with each other, with algaecide in the same bucket, or with acid. Add each one separately to the water with the pump running. Wear gloves and eye protection. No swimming until chlorine is back at or below the limit on your label. Poison Control: 1-800-222-1222.

FAQ

What kills black algae in a pool?

Chlorine kills it, but only after brushing breaks its protective layer. Brush each spot hard every day, raise chlorine with a product labeled for algae control at the amount its label gives, and keep brushing until the spots stop coming back.

Can black algae grow on a vinyl liner?

It can, but it is much less common than on plaster, because smooth vinyl gives it little to hold on to. On vinyl, check the spot test first: many dark spots are metal stains or growth behind the liner. Use only a soft nylon brush.

Why does black algae keep coming back?

Its anchors stay in the pores of plaster and grout after the surface is cleared. If treatment and brushing stop too early, it regrows in the same spots. Keep brushing those areas for several weeks and hold FC at the minimum for your CYA.

Sources

Orenda, Understanding algae (black algae as a cyanobacterium with a chlorine-resistant layer): blog.orendatech.com (a chemical maker's blog)
Taylor Technologies, K-1005 instruction card, trouble prevention chart (brush type by surface): PDF
Pool & Spa Marketing, Advice for service pros dealing with pesky vinyl liner stains (vitamin C test for metal; black algae locations on vinyl; stains from behind the liner): poolspamarketing.com (trade magazine)
Hayward, AquaRite manual (stabilizer 60–80 ppm for salt cells): PDF
CDC, Model Aquatic Health Code 2024, Annex (pH and chlorine effectiveness): cdc.gov (PDF)