What PoolBoss Says
Get rid of black algae by breaking its protective layer open before you dose anything. It roots into porous plaster under a waxy coating, so shock alone rinses over it. Brush each spot hard with a stiff brush, hold free chlorine at shock level for several days, then brush out the dead structure.
Dark spots the size of a pencil eraser on the deep-end steps, raised slightly off the plaster, and they do not come off the way everything else does. You brush, they look lighter, and a week later they are exactly where they were. That is black algae, and it is the one algae job where dosing harder does almost nothing.
The reason is structural rather than chemical. Black algae anchors roots down into the pores of the plaster and grows a waxy protective layer over the top, so the chlorine you add flows across a sealed surface instead of reaching anything alive. Break that layer and the same chlorine works. What follows is the sequence in order, how to tell it apart from the other two algae you actually see on a route, and what a pool on its third identical recurrence is really telling you about its plaster.
At a glance
Key takeaways
- Brush before you dose - black algae sits under a waxy protective layer, and chlorine added to an unbroken surface does essentially nothing.
- Use a stainless brush on plaster and gunite only; on vinyl or fiberglass it damages the surface, so use stiff nylon and expect a longer treatment.
- Drop pH to about 7.2 before shocking, because chlorine kills far faster in slightly acidic water than at 8.0.
- Shock immediately after brushing while the layer is open - brushing one day and dosing two days later wastes both steps.
- Plan on two to three visits over one to two weeks, and quote it that way rather than absorbing the return trips.
- Brush out the dead husks at the end, or a successful treatment still looks like a failed one to the customer.
- The same spots recurring for a third year is a plaster-porosity problem, and the documented history is what turns a resurfacing recommendation into a credible one.
How do I get rid of black algae in a pool for good?
Treat black algae as a physical problem first and a chemical one second. Unlike clearing a fully green pool, where raising free chlorine to roughly 40% of the cyanuric acid reading does the work on its own, nothing you add reaches a black algae colony until the waxy cap over it is physically broken open. Work the sequence in this order, and expect it to span two or three visits rather than one:
- Confirm it is black algae before you commit to the job. It shows as distinct dark spots with defined edges, raised slightly above the surface, and it resists a thumbnail. A stain is flat, sits in the plaster rather than on it, and does not lift no matter how hard you brush. Treating a stain as algae costs you a full shock cycle and gets you nowhere.
- Balance pH down to about 7.2 before you dose. Chlorine is a substantially more aggressive killer in slightly acidic water than at 8.0, and on a job where the chlorine only gets brief access through the openings you brush, that difference decides whether the exposed roots die or merely get inconvenienced.
- Brush every spot hard with a stiff brush, on plaster only. This is the step that does the actual work. You are not cleaning the surface, you are tearing the protective layer open so chlorine can get underneath. Expect to feel resistance, and expect the spots to look worse and more ragged afterward rather than cleaner.
- Raise free chlorine to shock level immediately after brushing, while the layer is open. Timing matters more here than on any other algae job. Brushing on Tuesday and shocking on Thursday gives the colony two days to reseal, and you have effectively done neither step.
- Hold the level for several days, not hours, and rebrush each time you are on site. Every rebrush reopens whatever has closed over and exposes the next layer down, which is why one visit almost never finishes it.
- Brush out the dead structure at the end. A killed colony leaves behind a dead husk still anchored in the plaster pore. It is no longer alive but it is still a rough, dark speck, and until it is brushed and filtered out the pool looks exactly like a failed treatment to the customer.
Black algae is rooted, not floating, and that changes everything
The single fact that decides this job is that black algae is not a film sitting on the surface. It puts root-like structures down into the pores of plaster and gunite, and it builds a waxy protective layer on top that is genuinely water-resistant. Chlorine is not selectively weak against it in any chemical sense. The chlorine simply never arrives, because it washes across a sealed cap while the living organism sits underneath in a pore it has colonized.
That is also why the three algae you actually meet on a route are three different jobs rather than three strengths of the same job. Green algae floats free in the water and dies at ordinary shock levels. Mustard algae clings to shaded surfaces, tolerates normal sanitizer levels, and survives out of water on damp equipment, which is why it travels between pools. Black algae does neither of those things - it barely spreads pool to pool - but it is by far the hardest to remove once established, because removal is mechanical before it is chemical.
The practical consequence is that identifying which one you are looking at determines the whole plan, including what you quote. A green pool is usually one visit and a filter run. A mustard outbreak is a chemistry job plus an equipment-sanitizing discipline across the rest of the pools on that day's run, because that organism moves on your net and brush. Black algae is a multi-visit mechanical job on one pool. Quoting any of the three as though it were another is how an algae recovery turns into unpaid labor.
| Type | How it looks | Where it lives | What actually removes it |
|---|---|---|---|
| Green | Cloudy green water, whole pool tinted | Free-floating in the water column | Shock to roughly 40% of the cyanuric acid reading, filter continuously |
| Mustard | Yellow dust, brushes away into a cloud and resettles | Shaded walls, steps, light niches; survives on damp equipment | Higher hold near 60% of cyanuric acid, plus sanitizing every tool that touched the water |
| Black | Distinct dark spots with defined edges, raised, resists a thumbnail | Rooted into pores of plaster and gunite, usually the same spots each time | Hard mechanical brushing to break the waxy layer, then a multi-day chlorine hold |
Brushing is the step that actually does the work
The brush decides this job, and the wrong brush wastes the entire chemical spend behind it. On plaster and gunite, a stainless steel brush is the tool that reliably opens the waxy layer; a standard nylon brush glides over black algae without breaking anything, which is why an operator can brush conscientiously every week and watch the same spots persist all season. That is the most common failure here, and it does not look like a failure while it is happening.
Surface type is a hard limit, not a preference. Stainless bristles belong on plaster and gunite only. On vinyl they will cut the liner, and on fiberglass they will scratch the gel coat, and both of those are damage you own. On those surfaces you use a stiff nylon brush and accept that the mechanical step is weaker, which is part of why the same treatment takes longer there.
Pressure and repetition matter as much as bristle choice. One firm pass is not enough on an established colony, and the honest expectation is that you are scrubbing each spot until it looks visibly disturbed and ragged rather than tidy. A spot that still looks smooth and intact after brushing has not been opened, and the shock you are about to add will run straight over it exactly as it did last month. Budget real time for this: on a pool with 15-20 spots across the steps and a shaded wall, the brushing alone adds something like 20-30 minutes to the stop, and it adds it on every visit of the treatment rather than just the first.
Holding chlorine for days, not hours
A black algae treatment is measured in days and visits, which is the part that has to be priced and communicated before you start. A green pool clears on a single heavy dose and a long filter run. Black algae needs free chlorine held at shock level continuously while you repeatedly reopen the colony, and on a residential route where you are on site once or twice a week that realistically means two to three visits spread over one to two weeks before the spots are gone and stay gone.
Practically, that means the dose is not a single event. Chlorine gets consumed, sun burns off what is left, and a pool left to drift back down to a normal level between your visits gives the colony room to reseal and recover. Where a customer can run the pump and is willing, having them maintain the level between stops shortens the job considerably. Where they cannot, the treatment simply takes more visits, and that belongs in the quote rather than in your unpaid time. It is worth confirming the pool is not fighting you chemically at the same time - a high stabilizer reading raises the shock target substantially, and it is worth checking whether phosphates are actually feeding the algae before assuming the hold is the only variable.
Set the customer's expectation on appearance too, because this treatment looks bad in the middle. The spots go grey or brown as the colony dies rather than vanishing, dead material sloughs into the water, and the pool commonly runs cloudy for a day or two while the filter catches it. A customer who was not told that reads the ugliest day of a working treatment as proof it failed, and calls you about it.
When it keeps coming back, the plaster is the problem
A pool that grows black algae in the same spots year after year, despite treatments that were done correctly, has a surface problem rather than a chemistry problem. Black algae needs somewhere to anchor, and aged plaster gives it exactly that: as the surface etches and its pores open, it stops being a smooth barrier and becomes a rooting medium that a brush physically cannot reach into. On a sound, tight surface the same organism struggles to hold. This is why the recurrence question and the plaster question are the same question.
Take a solo operator running 52 pools around Tucson and Oro Valley. One 1990s plaster pool in Oro Valley grows the same black spots on the deep-end steps every summer, and has for three years running. Each time it gets a full correct treatment: brush, hold, rebrush, clear. Each occurrence runs roughly $40 in chemicals and 30 extra minutes per visit across two or three visits. Three summers of that is not a run of bad luck, it is a recurring cost the operator is absorbing on a pool whose surface stopped being defensible some time ago.
The honest read, and the more valuable conversation, is that the customer needs a resurfacing quote rather than a fourth treatment. That conversation only lands with evidence, which is where the record does the work no memory can. A note and a photo of the same steps in three consecutive summers, sitting on that pool's own history, turns an opinion into a documented pattern - the visits are timestamped, the spots are visible, and the readings show the pool was not neglected between them. Whether you log the readings and the spots at every visit on paper or in software, that trail is what separates a credible recommendation from a sales pitch. It is also worth being deliberate about the readings worth logging at every stop, because a history that captures only what was dosed cannot show that the chemistry was in range while the algae came back anyway.
Resurfacing is a real expense and plenty of customers will decline it, which is a legitimate answer. What changes is the framing: once the pattern is documented, the recurring treatment becomes a known annual line item both of you have agreed to rather than a service failure the customer quietly blames on you.
FAQ
Frequently asked questions
How do I tell black algae from a stain or mold?
Use a thumbnail and a brush before you spend anything. Black algae sits slightly proud of the surface, has defined edges like a scattering of dark specks, and resists being scraped - a thumbnail may lift a little of it, and hard brushing visibly disturbs it. A stain is flat and part of the plaster: it will not lift, it often has a softer or more diffuse edge, and no amount of brushing changes it, which is the clearest single test. Metal staining from copper or iron also tends to spread across a wider area with a rusty or blue-green cast rather than forming discrete raised spots. What people call mold in a pool is almost always either black algae or, in the case of a pinkish or clear slime around fittings and under ladders, a bacterial growth that wipes away easily and does not root. The practical rule is simple: if brushing hard visibly changes it, treat it as algae, and if brushing does nothing at all, stop dosing and investigate it as a stain, because the treatments are completely different.
Can I put a chlorine tab directly on a black algae spot?
It works mechanically and it carries a cost you should count first. Rubbing a trichlor tab directly onto a brushed spot puts undissolved chlorine in direct contact with the exposed colony, which is genuinely effective on a handful of stubborn spots that keep surviving the general hold. The problem is that every trichlor tab also adds cyanuric acid, and cyanuric acid does not leave the water except by dilution. On a pool already sitting high on stabilizer, spot-treating repeatedly pushes the shock target higher and higher until you are dosing large amounts of chlorine to reach a level that used to be routine - you can end up making every future treatment on that pool harder in exchange for clearing a few spots today. Use it as a targeted tool on a small number of holdouts, check the stabilizer reading before you start, and stop reaching for it on any pool that is already elevated. Never leave a tab resting on a plaster surface unattended either, because prolonged contact will bleach and etch the spot it sits on.
Do I need a copper algaecide, or is chlorine enough?
Chlorine held properly does the killing, and a copper algaecide is a targeted follow-up on a pool that has genuinely resisted it. The order matters: an algaecide added to an unbrushed colony faces the same waxy barrier chlorine does, so it will not rescue a treatment that skipped the mechanical step. Where copper-based products earn their place is on a pool that has come back despite correct brushing and a correct hold, because copper is effective against black algae specifically and persists in the water longer than chlorine does. The tradeoff is real and worth stating to the customer: copper can stain plaster, particularly if the pool runs at a low pH or the product is overdosed, and it does not leave the water on its own. Quaternary ammonium algaecides are gentler on the surface but noticeably weaker against black algae, and they foam if overdosed. Cost is a factor too, since a treatment-strength algaecide typically runs $20-$40 a bottle. Treat it as insurance on a problem pool rather than a standard part of every job.
How long should the pool stay closed to swimmers?
The practical guidance is that swimming waits until free chlorine comes back down to a normal range, which after a multi-day shock hold usually means several days rather than overnight. During the treatment the pool is deliberately being held well above ordinary levels, and that is the reason to keep people out - high free chlorine is uncomfortable on skin and eyes and hard on swimwear. Because a black algae treatment holds that level across visits rather than for a single night, the closure is meaningfully longer than the customer expects from a standard shock, and telling them that at the start avoids a call mid-treatment. Test before giving any all-clear rather than going by the calendar, since how fast the level falls depends on sun, bather load, and stabilizer. Where a family needs the pool by a specific date, it is better to schedule the treatment around that than to cut the hold short, because a shortened hold is the most reliable way to be back treating the same spots in a few weeks. Any question about a specific health condition or sensitivity belongs with the customer's doctor and local health guidance, not a service call.
Can black algae grow in a vinyl or fiberglass pool?
It is far less common there, and when something that looks like it appears, it is usually worth a second look. Black algae needs a porous surface to root into, which is why plaster and gunite pools are its natural home and why it concentrates in aged, etched surfaces. Vinyl and fiberglass are effectively non-porous, so there is much less for the organism to anchor in. It can still take hold where the surface is compromised or where there is somewhere to hide - a wrinkle or seam in a liner, a scratched patch of gel coat, around fittings and steps, or in the tight gap behind a ladder. When it does, treatment is the same sequence with one hard constraint: no stainless brush, because it will cut a liner and scratch gel coat. Use a stiff nylon brush, accept that the mechanical step is weaker, and plan on more visits. On those surfaces it is also more likely that what you are seeing is a stain or a mark in the material itself, so run the brush test before committing to a treatment.
Does black algae mean the pool has to be drained or acid-washed?
Almost never on a first or second occurrence, and draining should be close to a last resort. A correct in-water treatment - brush, hold, rebrush - clears the large majority of black algae on a sound surface without touching the water level, and draining a pool carries real risk that has nothing to do with algae: an empty plaster pool can pop out of the ground under hydrostatic pressure where the water table is high, and a drained vinyl liner can shrink and fail to reseat. An acid wash sits between the two and is a genuine option for a plaster pool where the surface is stained and roughened and repeated treatments have failed, because it removes a thin layer of the plaster along with whatever is rooted in it. It is not maintenance, though - each wash takes material off a surface that is already thinning, and a pool that has been acid-washed several times is closer to needing resurfacing rather than further from it. If a pool is at the point where draining seems like the answer, the conversation worth having with the customer is about the condition of the surface itself and what it would cost to renew it.
Should I charge extra for black algae treatment, or fold it into the visit?
Quote it as a separate recovery job, and quote it before the first brush. Black algae is the most labor-intensive algae job there is: it needs two to three visits, adds 20-30 minutes of hard brushing to each of them, and consumes meaningfully more chlorine than a routine stop. Folding that into a weekly rate priced for a 20-minute maintenance visit means working several hours for free and, worse, creates the expectation that recurrences are covered too. Most operators price it as a flat recovery fee covering chemicals plus the extra visits, or bill chemicals at cost with a service-call rate for each additional trip. Either structure is fine as long as the customer agrees to it in advance, because a bill that arrives after the work is where disputes come from. Where the pool has recurred multiple years, the more useful conversation is the one about the surface: presenting the documented history of what each recurrence has cost gives the customer a real choice between paying for the treatment annually and paying once to fix the cause.
