Black Algae in a Pool: What Actually Kills It

You notice a dark speck on the plaster near the steps. It's small, it's just one, and you have other things to do that afternoon, so you make a mental note and move on.

A few weeks later there are a dozen of them, and they aren't just on the steps anymore.

Black algae is the one pool problem that doesn't announce itself. Your water can look perfect and your chlorine can test fine while something takes hold in the surface of your plaster and slowly spreads. The longer it sits, the harder it gets to remove, and that isn't a figure of speech. What starts as flat specks ends up as raised heads with a hard cap you can feel under your hand.

The good news is that it's beatable at any stage. You just have to know which stage you're at, and you have to stop treating it like regular algae.

How to Know You're Looking at Black Algae

Before you spend money on algaecide, make sure that's really what you're dealing with. Black algae behaves differently from every other algae you'll meet in a pool, and three things give it away.

It Doesn't Brush Off

Green algae comes off the wall in a cloud when you brush it. Mustard algae lifts into a dusty haze and drifts. Black algae does neither. You can put real weight behind the brush and the spot will still be sitting there once the water settles.

That one observation rules out most of what else it could be.

Your Water Is Still Clear

Black algae doesn't cloud your water, which is why people live with it for months without doing anything about it. You can have water you're proud of, a chlorine reading you're happy with, and spots spreading across the deep end wall the entire time.

It Keeps Coming Back to the Same Spot

This is the one that usually settles it. You brush, you shock, the spot fades, and you figure you're done. A week or two later it's back in exactly the same place, not a few inches over.

Nothing else in a pool behaves that way, and the reason it does is the same reason brushing alone won't fix it.

What Black Algae Looks Like as It Gets Worse

How fast a pool moves through these stages has very little to do with the calendar and almost everything to do with your chlorine. A pool that keeps bottoming out can go from a single speck to spots everywhere inside a week. A well-kept pool that picked up a little contamination somewhere will creep along slowly, and you might not see much change for weeks.

Either way, working out where yours sits is worth the two minutes. The stage decides how much work you're in for, and whether chemicals on their own are going to be enough.

A handful of small dark dots, each one separate, sitting flat against the plaster. Get close and you'll notice they aren't really black at all. Black algae is a very dark green, dark enough that it reads as black from a few feet away, and that's worth knowing because it's one of the few things you can check without a test kit.

The first ones usually turn up near the steps or tucked into a corner. At this stage it's easy to mistake for dirt or a bit of debris that settled, and even easier to talk yourself out of worrying about it.

This is the stage you want to catch it at. There's no real thickness to it yet, so your chlorine can still reach the algae once you've opened the spots up.

Now there are dozens instead of a handful, and they've moved well past wherever they started. You'll see them running down a wall, scattered across the floor, following the deep end. Some of the original spots have widened, and if you catch them in the right light a few are starting to look slightly proud of the surface.

How long that took tells you something useful. If it went from one spot to dozens inside a week, your chlorine has been bottoming out between doses, and that's the real problem to solve before anything else. If it crept along slowly over a month or more, your chlorine is probably doing its job most of the time and the algae is just holding on in the plaster faster than the water can kill it.

Nothing about the approach changes at this stage. It just takes more passes, over more square footage, and you have to keep at it longer than feels reasonable.

Leave it untreated for a few more weeks and the spots stop being flat. They build into raised heads with a hard cap over the top, and you can feel every one of them when you run a hand along the wall.

This is the stage where a brush stops being enough on its own, and where most people give up and call someone. You don't have to, but you do have to change your approach.

How It Got Into Your Pool, and Why It Stuck Around

Look this up and you'll get the same two answers almost everywhere. Somebody swam in a lake and carried it home on a swimsuit, or you have a dead spot behind the ladder where the water doesn't move. Neither one is wrong, exactly, but neither one explains much either.

Plenty of people swim in a lake on Saturday and their own pool on Sunday. Almost every pool has a corner the return jet never really reaches. If that were all it took, black algae would be in half the pools on your street, and it isn't. There are two separate questions worth pulling apart here: how the spores got into your water in the first place, and why they were still alive long enough to dig into your plaster.

Spores Have to Come From Somewhere

Your own equipment is the vector most people never think about. A brush, a pole, a net, a vac head — anything that goes in the water can bring something in with it. If you brushed your pool while it was green last summer and then hung the brush on the fence without rinsing it, that brush has been carrying whatever was in your pool ever since. The same goes for a brush you borrowed from a neighbor, or one that's been sitting damp in the side yard between uses.

Hosing your tools off and standing them in the sun to dry is not enough, which is the part most people get wrong. Mustard algae in particular handles drying out and full sun without much trouble, and that's exactly why it keeps turning up on equipment that looks perfectly clean. What kills it is chlorine. Pour a little liquid chlorine over the brush head and the bristles, or stand your tools in a bucket of water with a good splash of it mixed in, give it a few minutes, and rinse. It's worth making that a habit any time you've worked on a pool that had something growing in it.

Fill water is the other one worth checking. If your pool is topped off from a well, that water arrives untreated. City water shows up with a chlorine residual already in it, which is doing you a quiet favor every time you run the hose. Well water shows up with whatever the ground put in it, and that usually includes minerals like iron and manganese. Those minerals matter here for a reason that has nothing to do with staining: they consume chlorine. You can be adding a normal amount and still end up with less in the water than you think, which gives anything that came in along with it room to settle.

The swimsuit story does happen. It's just rarely the interesting part of the answer, because a pool holding chlorine kills what comes off a swimsuit without you ever knowing it was there.

Arriving Is the Easy Part

Spores land in pools constantly. They come off tools, off swimmers, out of the hose, out of the air after a storm. In a pool that's holding chlorine, they die before they ever amount to anything, and you never find out how many showed up.

Black algae needs a window. It needs your chlorine to sit at or near zero long enough for something to settle on a surface and get established, and that's a longer conversation than "I forgot to add tabs one week." The version I see most often is a pool that went green and stayed green for a while. Maybe it was a vacation, or a pump that quit, or a tab feeder that ran empty in August without anyone noticing. Eventually the owner shocks it hard, the water clears up in a couple of days, and the whole episode gets filed away as solved. But for those two or three weeks at zero, everything in that pool was free to settle wherever it wanted, and some of it settled into the plaster where shocking the water afterward doesn't reach.

This is also where the dead spot advice finally earns its keep, just not for the reason it's usually given. There's nothing special about still water. What matters is that fresh chlorine never really gets there. Your test kit is reading whatever is in the middle of the pool, and that number can look fine while the corner behind the ladder is running on far less than the rest of the pool ever sees. It isn't a circulation problem so much as a chlorination problem in one small place.

When a Salt Pool Quietly Stops Being a Salt Pool

Salt pool owners get caught by this one more than anybody, because the whole appeal of a salt pool is that the chlorine takes care of itself.

It does, right up until the salt level drops, and there are more ways for that to happen than people expect:

• Heavy rain dilutes it
• Splashout carries it out of the pool
• Backwashing sends some down the drain
• Draining and refilling resets it entirely

When the salt gets low enough, the cell produces less chlorine, and lower still, it stops producing altogether. An aging cell or one that's scaled up will do the same thing on a perfectly normal salt level.

Here's the part that catches people. At that moment you don't really have a salt pool anymore. You have an ordinary chlorine pool that nobody is adding chlorine to. Nothing about the pool looks different, the pump sounds the same, the water is still clear, and there's no reason to walk outside and suspect anything. A pool can sit like that for weeks, which is exactly the kind of window black algae is waiting for.

Test your salt with a strip or a meter rather than trusting the number on the control panel, since panel readings drift as the cell wears. Most systems want to be somewhere around 3,000 ppm, but check what yours asks for, because the target varies by manufacturer.

Why Brushing Alone Never Works

This is the answer to the spot that keeps returning to the same square inch.

Black algae isn't really algae at all. It's a cyanobacteria, and the practical difference for you is that it builds itself a roof. The colony secretes a sticky, gel-like coating over the top of itself, and that coating is what your chlorine hits instead of the organism. You can hold the pool at a textbook chlorine reading for a solid week and most of it will never reach anything alive.

Your plaster is the other half of the problem. Plaster looks smooth but it's full of tiny pores and hairline cracks, and that's where this stuff prefers to live. You'll read in a lot of places that black algae puts down roots. It doesn't, not in any literal sense, but the word stuck because the result looks the same from where you're standing. It settles down into the surface, below the level a brush ever really touches, and what's sitting down there is sheltered whether the coating on top is intact or not.

Put those two things together and brushing turns into something other than cleaning. When you brush a black algae spot you aren't scrubbing it off the wall. You're tearing the roof off so the chlorine already in your water can finally get at what's underneath. The spot fading is a side effect of that, not the goal. It's also the whole reason it comes back. Skip a few days and whatever survived down in the pores rebuilds the coating, and you're starting over in the same place, because that's where it has been the entire time.

The reverse is just as true, and it's the part that wastes people's summers. Brushing a pool with nothing in the water does close to nothing. Dumping chemicals into a pool you aren't brushing does close to nothing. Neither one is a treatment on its own, and doing them in the same week but on different days is most of why people think they've tried everything.

Your brush matters more here than it does for anything else you'll deal with.

• Plaster or gunite: stainless steel, or a combo brush with mixed bristles. You want something that will actually bite into the surface.
• Vinyl or fiberglass: nylon only. A wire brush will cut a liner and there is no fixing that afterward.

You're unlikely to be in that second situation anyway, since black algae is overwhelmingly a plaster and concrete problem and a smooth liner doesn't give it much to hold onto.

Once you're at the raised-head stage, a brush stops being the right tool no matter what it's made of. The cap has hardened enough that steel bristles just ride over the top of it. What works is taking a scraper to the heads first and knocking them off the surface, then going back over the area with your steel or combo brush.

The thinking hasn't changed, you're still just trying to open it up. It takes a harder tool to do it at that point.

When the Plaster Is Too Far Gone to Save

There's a version of this where chemistry isn't the answer, and it's worth naming because no one selling algaecide is going to bring it up.

Plaster wears out. An old surface that's been etched by low pH, or one that's gone rough and chalky with age, has pores that are wider and deeper than they were when it was new. That gives black algae more places to sit and gives your brush less chance of reaching any of them. You can run a perfect treatment on a pool like that, watch the spots disappear, and see them back before the end of the season, not because you did anything wrong but because the surface itself has stopped cooperating.

If you've treated the same pool properly two or three times and it keeps coming back across the whole floor rather than in one corner, the surface is the thing to look at.

• Acid washing strips off the top layer and takes a lot of what's embedded in it along for the ride
• Sandblasting goes further
• Replastering starts you over completely

All three cost real money and all three mean draining the pool, so nobody arrives at them happily, but they're genuinely the fix in a way that another round of shock isn't.

One thing to rule out first. If a spot doesn't respond to anything, won't come off under a scraper, and never spreads the way the stages above describe, you may not be dealing with algae at all. Pool stains can look remarkably similar, and treating one as the other is an expensive way to learn the difference.

Damaged plaster fools people the same way, and it's the one nobody warns you about. A chip or a gouge in the surface reads as a dark spot from the deck, because what you're looking at is a shadow sitting in a low place rather than anything growing on the wall. People will throw shock at a spot like that for an entire season, going back and forth on whether it's black algae or an organic stain, when the answer takes two seconds to get. Put your hand on it. If you feel a dent, you've found damage, and no amount of chlorine is going to fill it back in.

The Week That Actually Kills It

Almost every guide you'll find treats black algae as one problem with one answer, and that answer is always to shock the entire pool as hard as you can. Sometimes that's right. Often it's overkill, and it costs you a week of swimming and a pile of chemicals you didn't need to buy.

What actually matters is matching the method to what you're looking at. Four spots on the floor of the shallow end is a different job from spots running down every wall, and there's no reason to treat them the same way. Everything below builds on the same idea from the last section: you're opening the surface up so chlorine can reach what's underneath, and the only question is how much water you need to make hostile in the process.

Two Numbers You Need Before You Start

Almost every guide opens by telling you to balance the whole pool first. Skip that for now, for reasons I'll get to at the end of this section. There are only two readings you actually need in hand before you do anything.

The first is your stabilizer. Cyanuric acid protects chlorine from sunlight, but it also holds onto it, so the more you have, the more chlorine has to be in the water to get the same result. You cannot pick a shock target without this number, which is why "shock it four times harder" is such a useless instruction.

The second is your pH, and here you want one quick knockdown rather than a careful balancing act. Chlorine sits in your water in two forms, and the lower, more acidic end of the range keeps far more of it in the form that actually kills things. The same amount of chlorine at 7.2 does noticeably more work than it does at 7.8. Getting pH down before you dose is the cheapest improvement available to you all week, and it takes one dose of acid. Leave your alkalinity and calcium alone for now. You'll square those away at the end, and there's a reason for that I'll come back to.

The Algaecide Argument Has One Real Answer

Search for a treatment order and you'll run straight into a contradiction. One major retailer tells you to add algaecide two to three days before you shock. Another tells you to shock first and wait at least twenty-four hours before the algaecide goes in. Both of them sound certain.

The disagreement is real, and it comes from something true: a lot of algaecides are copper or quat based, and chlorine at shock levels will oxidize those products and burn them off before they do their job. Put them together at the wrong moment and you've wasted the bottle. The two retailers are solving that same problem from opposite directions, which is why their advice looks irreconcilable.

Here's the part neither of them says. Not every algaecide behaves that way. The product I use is made by SpecChem and the label says it can go in at any point in the process, chlorine level notwithstanding. So the right answer isn't to pick a side in an argument between two stores that don't know which bottle is in your garage.

While you have the bottle in your hand, look at the dose and stay inside it. Algaecide is not a product where more is better. Copper-based ones in particular will stain your plaster if you overshoot, which is a memorable way to trade one dark spot for another.

A Few Spots on the Floor

If you've caught it early and you're dealing with a handful of spots sitting on a flat floor, you can treat just that area and skip shocking the pool entirely. This is the first thing I reach for on a pool in that shape, and most of the time it's the only thing I end up needing.

Turn the pump off first, and then actually wait. Circulation keeps moving for a while after the motor stops, and if you start pouring before the water has gone still, all you've done is shock your pool the ordinary way and spread the shock everywhere except where you wanted it. Give it time to settle.

Once the water is calm, take granular shock and spread it evenly over the area with the spots in it. The instinct is to try to land grains directly on top of each spot, and that isn't the goal. As the granular sinks and starts dissolving, a thin layer of calcium and chlorine settles out and hovers right at the bottom over that patch of floor. That blanket is what does the work. You're building a pocket of extremely concentrated chlorine and letting it sit directly on the surface, which is something you cannot do by dosing the whole pool.

Give it two to five minutes. That's long enough for the granular to sink, dissolve and sit on the spots, and you don't need to stand there any longer than that. Then brush the area and start the pump back up.

• Not on a vinyl liner. Undissolved granular sitting on vinyl will bleach and damage it.
• Careful on colored or darker plaster, for the same reason.

A Spot or Two on the Wall

Walls are the awkward case. Granular shock needs somewhere flat to land, and a vertical surface gives it nothing to sit on, so the method above just doesn't apply.

What works instead is a trichlor puck. Take one, hold it against the spot, and rub it into the surface directly. You're doing the same thing you'd be doing with any other method, just by hand and in one small place.

Wear gloves, because trichlor is rough on skin.

The catch is scale, and this is where the advice you'll see online falls apart. People hand out the puck trick as a general answer to black algae, and if your walls are covered, picture what that actually means. You, in the deep end, holding your breath, scrubbing a chlorine tab across spot number forty, surfacing, going back down for forty-one. Nobody has ever finished that job. You'd burn a whole Saturday and your shoulders would give out long before the pool did.

Use a puck when you've got one spot, or a couple, in a place you can comfortably reach. The moment it becomes a list of spots, you're on the wrong ladder rung and it's time to treat the water instead.

Slugging the Shallow End

There's a liquid version of the same idea, and if you've ever added muriatic acid by pouring it in a slow column over one spot, you already know the technique.

With the pump off and the water still, pour liquid chlorine slowly and steadily over the affected area and let it sink. It holds together on the way down and lands as a concentrated slug right on top of what you're trying to kill, the same way acid does.

The catch is depth. This only works somewhere shallow. Try it over the deep end and the chlorine has too much water to travel through, so it disperses on the way down and arrives as nothing in particular. At that point you've just added chlorine to your pool, which is fine, but it isn't a treatment.

Turning the Whole Pool Into a Tub of Bleach

Some situations don't leave you a clever option:

• Spots running down the walls
• A floor that's covered rather than speckled
• A deep pool, where granular dissolves and drifts off long before it reaches the bottom

When the problem is that widespread, the pool itself becomes the treatment.

Your stabilizer reading sets the target.

• CYA 30 to 50: take the pool to 30 ppm of free chlorine on day one
• CYA well outside that range: go higher, somewhere in the neighborhood of 40 to 50 ppm

The reason for that second one is that more of your chlorine is being held by the stabilizer instead of working.

This is also the week to stop using test strips. Most strips only read up to about 10 ppm of free chlorine, and past that they bleach out and hand you a pale square that can look like a low reading or no reading at all. You can be sitting at 35 ppm and get a result that tells you to add more.

A Taylor kit with the drop-count chlorine test doesn't have that ceiling, because you're counting drops until the color changes instead of matching a pad against a chart. If you're going to aim for a specific number this far above normal, you need a test that can actually see it.

This is a multi-day job, so it helps to think about it as days rather than as one big push.

• Scrape the heads off first if you're at the raised-head stage
• Brush the entire pool before you add anything: walls, floor, steps, the corners behind the ladder, every spot you can find and the area around it
• Bring the pool up to your target chlorine level
• Add your algaecide wherever your label puts it in the order
• Start the pump and leave it running around the clock until you're finished

The brushing is the half of the day that decides whether any of the rest worked. Chlorine only does something on surfaces you've opened up, and running the pump nonstop is what carries it into every corner instead of leaving it where you poured it.

• Look at the pool before you test anything, and look for visible spots rather than clear water
• Brush everything again, exactly the way you did on day one
• Test your free chlorine
• If spots are still there, bring the pool back up to the same target

That last one trips people up. You're topping back up to the same number, not stacking a bigger dose on top of yesterday's. A day of sunlight and a day of killing algae used some of what you added, and you're replacing it.

• Brush the whole pool
• Test your chlorine
• Check for phosphates and add remover
• Bring it back to target if you can still see anything
• Repeat tomorrow

That's the whole loop. Resist the urge to give yourself a deadline while you're in it. A pool with a few spots might be finished on day three, and a pool with covered walls can run a week or longer. The pool tells you when it's done, and it tells you by not having black algae in it. Stopping on a day you picked in advance is how this turns into an annual event.

The Days After the Last Spot

• Let your chlorine come down on its own instead of dropping it
• Keep brushing the areas that had spots while it does
• Vacuum the calcium residue off the floor if you used granular shock
• Rebalance the water once chlorine is back to normal, which is the next section
• Clean out your filter last, which is further down under prevention

The bottom three get skipped constantly, and they're the difference between finishing this once and finishing it again in a month. The survivors at this point are the deepest ones, and nothing about them is visible.

The residue is worth a word, because it surprises people. If you ran this treatment on calcium hypochlorite, you've been adding calcium to the pool every single time you dosed, and a week of that leaves a layer of it sitting on the floor. It's fine, it just needs to come out. If you used liquid shock instead, there's a good chance you won't have much of anything to deal with.

Vacuum that up before you clean your filter, not after, or you'll end up cleaning the filter twice. How you get it out depends on your equipment:

• Multiport valve: vacuum to waste, so the calcium never enters the filter at all
• Cartridge filter: no waste setting exists, so you'll need an external pump to move that water out, or a siphon if your pool sits high enough for gravity to do the work

• Keep granular in its container until the moment you're using it
• Add chemicals to water, never water to chemicals
• Never combine two products in the same bucket or the same pour

Rebalance After, Not Before

Here's the part I skipped at the top, and it's where most guides have the order backwards.

Setting a pool up perfectly and then running a week of treatment on it is wasted work. pH drifts upward over the course of a job like this. It isn't dramatic and you won't watch it jump, but across several days of dosing and running the pump nonstop it moves, and where you left it on day one is not where you'll find it at the end. Dialing it in before you start means doing that work twice.

There's a second reason not to chase it mid-week. At the chlorine levels you're running, a pH test isn't telling you much anyway, because high free chlorine bleaches out the indicator in the test. You'd be making decisions off a color that isn't reporting honestly.

So leave it alone while you work and do the real balancing once your chlorine is back to a normal range. Check your total alkalinity and get it into the 80 to 120 range, check calcium hardness while you're there, and bring pH down to 7.2.

That pH target is the one worth being deliberate about, because it's prevention and not cleanup. A pool sitting at 7.8 is running weaker chlorine than the same pool at 7.2, even though the chlorine test reads identically in both. You can hold a number you're happy with all season and still be handing algae more room than you think. Finishing low is how you make sure the pool you just cleaned stays that way.

Keeping It From Coming Back

The most common way people end up doing all of this twice is stopping the day the pool looks clean.

Give it about another week. Keep brushing the areas that had spots and keep your chlorine up on the high side of normal, even though there's nothing left to look at. Whatever was deepest in the plaster is the last thing to die, and it doesn't announce that it's still there. Quitting the moment the surface looks right is quitting with a few survivors still in the pores, and those are the ones that hand you the same spot in three weeks.

Then deal with your filter, which is the step almost nothing written about black algae bothers to mention. Everything you brushed off the walls went somewhere, and that somewhere is your filter media. Leave it sitting in there and you've built yourself a reservoir that can reseed the pool the next time your chlorine dips.

What that takes depends on what you have.

• Sand: a good backwash is enough
• Cartridge: a proper cleaning rather than a hose-off. Pull it and give it the full soak you'd normally save for an end-of-season clean.
• DE: both halves of the job. Backwash it, then pull the grids out, rinse them down and sanitize them at five parts water to one part bleach for about five minutes. Rinse them off and recharge the filter.

After that, pay attention to the exact places it was. Black algae comes back where it lived before, so those few square inches deserve a look every time you're out there with a brush. Catching a returning spot while it's still a speck means a puck and thirty seconds, not another week of your summer.

Everything else worth doing here comes down to one thing, and you already know what it is from earlier in this post. Your chlorine cannot be allowed to reach zero. Not for a week while you're away, not while a tab feeder sits empty in August, not for the stretch after a storm dumps six inches in the pool, and not quietly for a month because a salt cell stopped producing and nothing looked any different. Every one of those is a window, and black algae only needs one.

One last thing to watch if you run tabs. Trichlor adds stabilizer to your water every time it dissolves, so your CYA climbs all season without you doing anything. A chlorine reading that was genuinely fine in May is doing less work in August at the same number on the test. Check your stabilizer a couple of times a season, because a pool can drift into being under-chlorinated while the chlorine test keeps telling you everything is fine.

Black Algae Is a Consistency Problem

Go back through everything above and notice what all of it has in common.

The vacation week. The tab feeder that ran empty in August. The salt cell that quietly stopped producing while the pump sounded exactly the same. The stabilizer that climbed all season until a normal-looking chlorine reading wasn't doing normal work anymore. Not one of those is a knowledge problem. Nobody reading this needs to be told that chlorine matters. Every single one of them is a noticing problem, and they all look completely fine on the day they happen.

That's the thing a single test can't help you with. A test strip or a drop count tells you about right now. It cannot tell you that your free chlorine has been sliding for three weeks, or that you've topped off from the hose six times this month, or that your CYA has doubled since you opened the pool. Those are patterns, and patterns only exist if somebody wrote the numbers down.

This is the part where I tell you I built something for exactly this, because I did. I work on pools for a living, and the pools that get black algae are almost never owned by people who don't care. They're owned by people who tested on a Saturday, got a number they were happy with, and had no way to see that the same number had been quietly getting worse for a month.

FlowBeacon is where I put that. You log your readings, it keeps them, and it shows you the direction they're heading instead of making you hold three months of test results in your head. It runs in your browser and on your phone, it doesn't cost anything to start, and it doesn't delete your history the moment you've logged enough of it to be useful, which is the trick most free pool apps pull.

Start logging your pool for free and give it a season. The black algae you never have to treat is the one you'll never read an article about, and that's the entire goal.

You already know how to kill this stuff now. The scraper, the granular, the puck, the week of brushing, the ppm target for your stabilizer level. Keep that for the pool you have today. The better outcome is the one where you catch the drift before it ever opens the window.