Black Algae in a Pool: How to Kill It and Stop It Coming Back

Most pool problems announce themselves in the water. Black algae announces itself on the wall, and then refuses to leave. It arrives as a scatter of small dark spots in the places nobody looks first — the shaded corner of the steps, the seam behind the ladder, the rough patch around a light niche — and it survives treatments that would clear a whole pool of anything else.

The reason is that it is not really algae, and almost every mistake made with it follows from treating it as though it were. Ordinary green algae lives loose in the water and dies the moment the sanitiser level goes up. Black algae is a colony that anchors into the surface itself and builds a protective layer over the top of itself. Raise the chlorine as high as you like: it does not reach the cells that matter, because they are not exposed.

What follows is how to identify it correctly, the sequence that actually removes it, how to tell when the problem has stopped being about chemistry at all, and — for anyone running more than their own pool — how to keep one infected pool from turning into five.

What black algae actually is

Black algae

A colony-forming cyanobacterium rather than a true alga. It anchors into the pore structure of a pool surface and secretes a protective outer layer, which is why it survives sanitiser levels that clear ordinary algae and why it regrows from the same spots if the anchoring layer is left behind.

Cyanobacteria are among the oldest and most stubborn organisms on the planet, and the two traits that make them stubborn are exactly the two that matter in a pool. They build a protective outer layer over the colony, and they anchor into whatever they are growing on rather than floating free.

That anchoring is why the surface of the pool matters so much. Plaster, gunite, exposed aggregate and grout are all porous at the scale that matters here — a colony can get roots down into the pore structure and sit below the plane you can reach with a brush. Vinyl liners and fiberglass shells are far less hospitable, which is why black algae is overwhelmingly a concrete-and-plaster problem. It still turns up on those surfaces occasionally, but it does so at seams, behind fittings and in the rough spots where the smooth surface has been compromised.

The protective layer is why the pool can test perfectly and still have black spots on the wall. A sanitiser reading tells you what is available in the water. It says nothing about what is reaching the cells underneath a cap that was built specifically to keep it out.

Location is a clue in its own right. Colonies establish where circulation is poor and sunlight is weak: the shaded end, the underside of steps, corners the return jets do not reach, around light niches, behind a ladder. If a pool has a dead spot in its circulation, that is where you will find the first heads.

Telling it apart from what it gets mistaken for

The single most useful diagnostic is a thumbnail. Run it firmly over one of the spots. Green and mustard algae smear and lift; black algae resists, and if it does come away it tends to come away as a discrete piece rather than a smudge, often leaving a pale or discoloured mark underneath where the surface has been attacked. A stain does not come away at all, because it is in the surface rather than on it.

Getting this right before you start matters commercially as well as technically. Treating a mineral stain as black algae wastes a week of elevated chemistry on a problem chemicals will not touch, and treating black algae as a stain means an acid wash that leaves the colony alive on a surface that is now rougher and easier to colonise than it was before.

Why a normal shock does not touch it

The standard response to algae is to raise the sanitiser sharply and let it do the work, and for anything living loose in the water that is exactly right. Applied to black algae it produces a specific and recognisable failure: the water goes clear, the pool looks corrected, the spots are still there, and a fortnight later they are bigger.

Three things are happening. The cap is keeping the dose off the cells. The anchoring layer in the surface is untouched even if the visible head is damaged, so the colony regrows from below. And a single shock is a spike rather than a siege — the level rises, decays over a day or so, and is gone long before it has done sustained damage to something built to withstand it.

There is also a common hidden reason a correct-looking dose achieves nothing, and it is worth ruling out before you conclude the treatment failed. Cyanuric acid protects chlorine from sunlight, which is useful, but it also holds chlorine back from working. In a pool where stabiliser has been allowed to climb season after season — which happens quietly wherever stabilised tablets are the only sanitiser — the chlorine reading can look healthy while very little of it is active. A pool in that state will resist every algae treatment you try, black or otherwise, until the stabiliser comes down, and that means replacing water rather than adding a product.

The removal procedure, in the order that actually works

The order below is not interchangeable. Nearly every failed black algae treatment is the right steps performed in the wrong sequence — usually dosing first and brushing afterwards, which puts the chemical in the water at the one moment the colony is still sealed against it.

1. 1

   Confirm what you are looking at

   Run the thumbnail test on two or three spots in different parts of the pool. Note where they are: a scatter concentrated in one shaded corner is a circulation problem as much as a chemistry one, and that will matter later.

2. 2

   Balance the water and check the stabiliser first

   Get pH into range before dosing anything, because chlorine loses a great deal of its effectiveness at high pH and you will be relying on it for days. Test cyanuric acid at the same time. If it is high, dilution comes before treatment — otherwise you are about to spend a week proving that a sanitiser you have suppressed does not work.

3. 3

   Clean the filter before you start, not after

   The filter is about to be asked to run continuously and take a heavy load of dead material. Starting the job with a loaded element means poor circulation through exactly the days when circulation matters most.

4. 4

   Brush every spot hard, before any chemical goes in

   This is the step the whole job turns on. Short, hard, overlapping strokes with the right brush for the surface, working each head until the cap is visibly broken and the spot looks ragged rather than smooth. You are not trying to remove the algae — you are trying to open it.

5. 5

   Dose, and read the label rather than guessing

   Black algae needs a sanitiser level well above a routine shock, and the figure depends on the product, the volume of the pool and the stabiliser level you measured in step two. All three are knowable. None of them is a rule of thumb you can carry between pools, so work it out for the pool in front of you.

6. 6

   Treat the stubborn heads directly

   Individual heads that survive the first round respond to concentrated contact: a chlorine tablet held and rubbed directly onto the opened spot puts sanitiser against the colony at a strength the bulk water never reaches. Gloves and eye protection, and keep the tablet away from anything else you are carrying.

7. 7

   Hold the level, and run the pump around the clock

   This is the part that separates a treatment from a gesture. Test daily and top the level back up as it decays, keeping it elevated for several days rather than letting it fall back overnight. The filter runs continuously throughout.

8. 8

   Brush again every single day it is held

   Colonies rebuild their protective layer quickly. Daily brushing keeps them open for the whole period the sanitiser is elevated, which is the only way the dose reaches the cells that are actually keeping the colony alive.

9. 9

   Vacuum the dead material out, to waste if you can

   Dead colonies go grey or brown and crumble. Get that material out of the pool rather than through the filter where practical, clean the filter again once the job is done, and only then let the chemistry fall back to normal.

The brushing is the job

If there is one thing to take away it is this: brushing is not tidying up around the treatment, it is the treatment. Everything else — the dose, the days, the pump running — is there to exploit the opening that the brush creates. A pool that gets a textbook chemical treatment and one brush at the start will still have black algae in a fortnight. A pool that gets daily brushing and a level that is genuinely held will usually not.

This also explains why black algae is one of the few pool problems where a weekly service interval is not enough on its own. A colony rebuilds its cap in days, so a once-a-week visit brushes it open and then leaves it sealed for six days while the sanitiser you dosed decays. Either the visits go daily for the duration of the treatment or the customer is brushing between them, and which of those it is needs deciding out loud rather than assumed.

When it stops being a chemistry problem

There is a point at which more treatment is the wrong answer, and recognising it is what separates an operator who solves the problem from one who bills for it every summer. The test is recurrence in place. If a pool has had a genuine treatment — brushed open daily, level actually held, filter clean — and the spots come back in exactly the same locations within a season, the water is not what is feeding them. The surface is.

A plaster surface that has aged, been acid-washed a few times, or simply been in the ground for a long while is no longer smooth at the scale a cyanobacterium cares about. It is pitted and open, and a colony can anchor below the depth a brush reaches. At that point every treatment is knocking the tops off something whose roots are untouched, and the pool is on an annual cycle that chemicals cannot break.

The practical move is to document it rather than argue about it. Photograph the spots with something in frame for position, record the date, and do the same after each treatment. Two or three cycles of that and the pattern is a matter of record instead of an opinion, which is a far better conversation to have with an owner who is about to be told their pool needs resurfacing.

Keeping it off the rest of the route

On a single pool black algae is a difficult job. On a route it is a contamination risk, because the thing that moves it between pools is your own equipment. A brush, a pole, a leaf net, a vacuum head or a test kit that has been in an infected pool and goes into the next one without treatment is the most common way a single problem pool becomes a cluster of them on the same street.

The handling is not complicated, but it does have to be deliberate, because the default behaviour — one set of gear, used everywhere, rinsed when it looks dirty — is exactly the behaviour that spreads it.

Pricing the job and having the conversation

Black algae remediation is not maintenance and it should not be absorbed into a maintenance rate. It is a multi-visit job with daily attendance, a heavy chemical cost, and a real chance that it ends in a recommendation the owner will not want to hear. Treating it as something you fold into the weekly service is how a route loses money quietly and how the technician ends up resenting a stop.

That makes it one of the clearest examples of why the scope and exclusions in a service agreement matter. An agreement that says what routine service covers, and that remediation of an established algae problem is quoted separately, turns an awkward conversation into a paragraph both parties already agreed to. Getting that language right ahead of time is worth more than any negotiating position afterwards — the same principle that applies to every other exclusion worth writing into a pool service contract.

Two pieces of related work usually follow a black algae job and are worth planning for. The filter takes a heavy load through the treatment and will need proper cleaning rather than a rinse once it is over, which is a scheduled stop of its own rather than something squeezed into the last visit. And a pool that has been through several days of elevated chemistry and heavy brushing very often finishes clear of algae but hazy, which is a filtration and chemistry question rather than a sign the treatment failed.

Finally, be clear with the owner about which problem they have. Green water and black spots get talked about as though they are the same category of thing, and they are not: one is a sanitiser failure that a correct dose fixes in days, and the other is a colonised surface that takes a week of daily work and may not be fully solvable without addressing the plaster. The procedure for a green pool is a genuinely different job, and saying so up front is the difference between a customer who understands what they are paying for and one who thinks they were overcharged for something that should have taken an afternoon.

The habit worth building is looking at the walls, not just the water. Black algae is far easier to remove when it is three heads in a shaded corner than when it is a scatter across a whole end of the pool, and the difference between those two states is usually a few weeks of nobody glancing at the steps.