How to Get Rid of Black Algae

Why this one is different

Green algae floats and dies when you shock it. Mustard algae hides and comes back. Black algae does something neither of the others does: it builds a physical shield over itself and puts root-like structures down into the pool surface.

That shield is the whole problem. You can hold free chlorine at shock level for a week and the organism underneath is simply not in contact with it. This is the one case on this site where the chemical is genuinely not the answer - the brush is.

Confirming it is black algae

Before spending weeks on this, be sure. Our algae identification guide covers all three, and the short version for this one:

It is far more common in gunite and plaster than in vinyl, because it needs a rough surface to anchor into. If you have a vinyl liner and think you have black algae, it is much more likely to be a stain or mold on the liner surface.

The protocol

1. **Test and balance first.** pH into 7.2 to 7.6, because both chlorine and copper behave badly outside it - see how to balance pool water. Copper in particular stains at high pH, and a copper algaecide is likely part of this treatment.
2. **Brush each spot hard with a stainless brush.** This is the treatment step, not preparation. The goal is to break the protective crust, not to remove the spot - it will not come away in one pass and it does not need to. Use a small detail brush so your force concentrates on the spot rather than spreading across a wall.
3. **Apply algaecide directly to the brushed area.** Not broadcast across the pool. Some owners turn the pump off, let the water still, and apply the treatment right at the spot so it sits there rather than dispersing.
4. **Hold elevated free chlorine** while you work through it. See the dose tables for your volume. Chlorine cannot penetrate an intact crust, but it finishes what the brushing exposes.
5. **Run the filter continuously** and clean it after each round. You are putting a lot of organic material into suspension.
6. **Repeat every two or three days.** Black algae takes weeks. Anyone promising a weekend fix is describing a different problem.

Copper or polyquat

This is the one real product decision, and both sides have a genuine case.

The pragmatic answer for most pools: copper to break an established infestation, then polyquat to hold the ground you took. Using copper indefinitely is how people end up with metal staining, which is a slower, more expensive problem than the one they were solving.

The picks

Top pick - Spot-treating black algae

Poolmaster 5-Inch Stainless Steel Pool Algae Detail Brush

The narrow brush that reaches into the grout lines and corners where black algae actually anchors.

The published figure

5 in stainless steel bristles on an aluminum back, for gunite and concrete

An 18 inch wall brush sweeps a wall. It cannot put meaningful pressure into a two-inch patch in a grout line, and pressure is the entire mechanism against black algae.

Black algae grows a protective layer that chlorine cannot penetrate. Breaking that crust is what lets any treatment reach the organism, and a small head concentrates your force on the spot rather than spreading it over a wall.

Stainless is for gunite, concrete and plaster only. On a vinyl liner it will cut.

What it does well

• Concentrates pressure on a small area, which is what the job needs
• Reaches grout lines, corners and steps a wide brush skims over
• Costs a fraction of the chemicals it makes work

What it costs you

• Useless for routine wall brushing
• Stainless will damage a vinyl liner

Skip it if your pool has a vinyl liner. Black algae is rare on vinyl and stainless will cut it.

$13.13 - View on Amazon

#ad · how this is funded · price as of October 9, 2026

Pick 2 - Black algae in plaster

In The Swim Black Algaecide, copper sulfate pentahydrate, 1 quart

The right chemistry for black algae and an actively bad idea for anything else, which is exactly how the maker positions it.

The published figure

Copper sulfate pentahydrate, sold specifically for black algae

Black algae anchors into plaster and grows a protective layer that ordinary chlorine cannot penetrate. Copper is one of the few actives that reaches it, which is why this product exists as its own SKU rather than a general algaecide.

Brushing is not optional here. A stainless brush breaks the protective layer so the chemical can reach the organism; without that step you are treating a shell.

Copper stains. Use it for the identified problem, then stop.

What it does well

• Purpose-made for the one algae type that resists chlorine
• Named single active ingredient, so you know what you are adding

What it costs you

• Copper staining risk on plaster and on light-colored liners
• Useless against ordinary green algae relative to shock
• Requires aggressive brushing to work at all

Skip it if you have not positively identified black algae. Green spots that brush away easily are ordinary algae and need shock, not copper.

$24.99 - View on Amazon

#ad · how this is funded · price as of October 9, 2026

Pick 3 - Black algae without copper

Pool Mate Premium Black Algaecide, 1 quart

A 60% polyquat that names black algae and publishes its dose rate, which is the copper-free route to a problem usually treated with copper.

The published figure

60% polyquat; listing states 4 fl oz per 10,000 gallons weekly and names black algae specifically

Most black-algae products are copper-based, and copper stains. This is polyquat at the same 60% concentration as a general-purpose preventative, marketed at black algae specifically.

The listing states 4 fl oz per 10,000 gallons weekly. That is a real, checkable rate, and it lets you work out what a quart costs to run before you buy it - a quart is eight weekly doses for a 10,000-gallon pool.

It still will not work without brushing. Nothing reaches black algae through its protective layer, whatever the active ingredient.

What it does well

• 60% polyquat, the top of the retail concentration range
• Publishes a dose rate, which most of this category does not
• No copper, so no staining risk on plaster

What it costs you

• Weekly dosing is an ongoing cost
• Polyquat is slower against established black algae than copper is

Skip it if the black algae is well established in rough plaster. Copper reaches it faster, and you can manage the staining risk by watching pH.

$30.74 - View on Amazon

#ad · how this is funded · price as of October 9, 2026

Pick 4 - Brushing before you shock

Poolmaster 18-Inch Stainless Steel Pool Algae Brush

Stainless bristles on a concrete pool are the step that makes the shock work, and the cheapest item in any of these roundups.

The published figure

18 in stainless steel bristles on an aluminum back, for gunite and concrete

Algae clinging to a wall is physically shielded from the water around it. Brushing first is what lets chlorine reach the organism rather than the film over it, which is why every ordered fix on this site puts brushing before dosing.

Stainless is for gunite, concrete and plaster only. On a vinyl liner it will cut, and you want nylon.

18 inches is the size that covers a wall in a reasonable number of passes without being unmanageable on a long pole.

What it does well

• Makes the shock dose that follows it materially more effective
• Costs a fraction of any chemical it improves
• Aluminum back resists the corrosion that kills cheap brushes

What it costs you

• Stainless bristles will damage a vinyl liner
• Pole sold separately

Skip it if your pool has a vinyl liner. Buy the nylon equivalent instead.

$21.17 - View on Amazon

#ad · how this is funded · price as of October 9, 2026

Pick 5 - Clearing an algae bloom

In The Swim Pool Shock, 68% Cal-Hypo Granular, 25 lb

The highest available-chlorine percentage in common retail cal-hypo, sold loose so you dose by weight instead of by packet.

The published figure

68% available chlorine, calcium hypochlorite, sold loose by the pound

68% available chlorine means 1 lb raises 10,000 gallons by about 8 ppm. A green pool typically needs 10 to 30 ppm depending on how far the bloom has gone, which is 1.2 to 3.7 lb per 10,000 gallons.

Loose granular is the format that lets you weigh a dose. Pre-measured 1 lb packets are convenient and fine for weekly maintenance, but they force you to round a shock dose to the nearest pound, which on a small pool is a 30% error.

Cal-hypo adds calcium hardness and no cyanuric acid. That is the right trade for a bloom and the wrong one for a pool that already sits above 400 ppm calcium.

What it does well

• 68% is the top of the common retail range, so you carry and store less product per ppm
• Loose granular lets you weigh an exact dose rather than round to a packet
• Adds no cyanuric acid, so shocking repeatedly does not drive you into chlorine lock

What it costs you

• Adds calcium hardness with every dose
• Must be pre-dissolved in a bucket for vinyl liners, which the label states
• A 25 lb bucket is more than a small pool will use in a season

Skip it if your fill water is already hard, or you own a small above-ground pool that will never work through 25 lb before the bucket cakes.

$109.99 - View on Amazon

#ad · how this is funded · price as of October 9, 2026

When to stop treating and fix the surface

If the spots come back in exactly the same places after several full rounds, the chemistry is not what is failing. Black algae anchors into surface defects, and plaster that is rough, pitted or eroded in those spots is sheltering it from everything you apply.

At that point the durable answer is surface repair or resurfacing. That is a real, expensive answer and it is genuinely better than a fourth season of algaecide - which will cost more in total and will not work.

We do not cover resurfacing on this site and would not pretend to. It is a trade job, and it is the correct referral rather than something to talk you out of.

Preventing the return

• **Keep free chlorine steady, not spiky.** Black algae establishes during the gaps, and a pool that swings between 0.5 and 5 ppm has more gaps than one held at 2 to 3.
• **Watch cyanuric acid.** Above roughly 50 ppm your chlorine reading overstates what the chlorine is doing, which manufactures those gaps invisibly - see cyanuric acid.
• **Brush the rough areas weekly** even when nothing is visible. Grout lines and steps are where it starts.
• **Treat anything that entered another pool.** Black algae travels on floats, brushes and swimsuits the same way mustard algae does.
• **Dose a polyquat preventatively** once the water is clear and free chlorine is back to normal.