Pool Algae: What Actually Kills It (And Why You Probably Don't Need Algaecide)

Right now in your pool there's algae. Not a little, not somewhere out there, but in your pool right now. It's been there the whole time you've owned the thing. It's not doing anything. It's a spore mostly. A cyst. Basically asleep. Doesn't need much to stay alive, and it's very good at waiting. Weeks. Months. It's patient in a way nothing in your life is patient. And it's not spread out evenly. It's collected quietly. In the one corner of your pool where the water barely moves. Behind the ladder. Under the skimmer's blind spot. Wherever your return jets don't quite reach. That's not an accident. That's where it wants to be, because that's the one place your chlorine is the thinnest.

So it sits there, waiting on two things. A little bit of warmth and a gap, just a small one, between how much chlorine that corner actually has and how much it actually needs. Most days, it doesn't get both. Most days, nothing happens. Then, one day, it does. Boom.

Welcome to the Pools Scientific Podcast, elevated pool care through science and technology. I'm John Cooper, and whether you own a pool, service them, or run a pool company, this show is built for you. Real Chemistry, Real Answers. Every Tuesday.

It's not that algae suddenly showed up. It's that it was already doubling and you just couldn't see it yet. Algae reproduces by splitting itself in half. One cell becomes two. Two become four. Under good conditions, that whole process can happen about once per day. Once a day doesn't sound like much. It is though once you follow the math. Start with one cell. Double it. And then double the ones that it's created. Once a day for a week. And you have maybe a hundred and twenty cells. You'd never see that in a pool of any size. It's nothing. Keep doubling. By day ten, you're over a thousand, still invisible.

By day fourteen, over sixteen thousand. Still nothing you'd notice when you're walking by. Then a few more days pass, and somewhere around day sixteen or seventeen, you cross a line where the number of cells is actually enough to change the color of the water. And from where you're standing, it looks like it happened overnight. It didn't happen overnight. It happened exponentially. And I've seen this exact shape before. Not in a pool, behind about six feet of lead and steel working with a reactor vessel I couldn't see into, running the numbers from that same energy balance equation I've mentioned before. Twenty eight variables. Every one of them has to be correct.

Not just close. Correct. A reactor runs on neutrons. Every fission event releases more of them, and each of those either escapes, gets absorbed, or splits another atom and creates still more. Control rods, made of materials built specifically to absorb neutrons, boron, cadmium, hafnium, sit in the core to soak up just enough of that population to hold the reaction steady. Pull the rods out slightly, absorb a little less, and the population starts multiplying instead of holding flat. That shift from holding steady to multiplying has a name. It's called going supercritical. Supercritical doesn't announce itself. For a while, the power gauge barely moves. You'd swear nothing was happening.

Then the neutron population crosses a threshold and the needle moves so fast it looks like it jumped. It didn't jump. It was doing exactly what the math said it would do the entire time. Nobody who trusted the equation was ever surprised by that moment. Everybody who was watching the gauge instead of the math was.

One more piece of this matters for later. When a reactor needs to stop that reaction immediately, not slow it, but completely stop it, you scram the rods, driving every control rod fully into the core at once. It instantly floods the reaction with so much neutron absorption that the multiplication stops cold, dead cold, with no gradual decline. It's the nuclear equivalent of overwhelming force rather than gradual correction. Hold on to that idea. Later in the episode, when we talk about what it actually takes to kill an established algae bloom, you'll recognize the same move, just with chlorine instead of control rods. Your pool doesn't have a gauge.

It has a color. And by the time that color moves, the math already finished its work. The bloom isn't the beginning of the problem. The bloom is just the day you finally noticed it. Not all algae is the same problem. There are four species you'll actually run into and they don't play by the same rules. Green algae is the one everybody pictures. It's the most common and it's also the least defended. It floats free in the water instead of clinging to anything, which is exactly why it clouds the whole pool instead of just showing up in a corner. No real protective structure. Correct chlorine handles it cleanly, the way most people already assume chlorine handles all of these.

It's the one case where that assumption happens to be right. Yellow or mustard algae is genetically a cousin of green algae, not a different species entirely, but it evolved a protective outer layer green never bothered with. It doesn't float. It clings to walls, steps, behind ladders, or anywhere the water barely moves. It has a dusty yellow brown color and is easy to mistake for pollen or sand. You brush it off, it looks gone, and is back in a day or two, because normal maintenance chlorine levels genuinely aren't enough to kill it. It takes sustained elevated chlorine, not a quick dose, plus real brushing to break it loose so the chlorine can actually reach it.

Mustard has one more habit worth knowing about. Swimsuits, pool toys, brushes, vacuum heads, anything that goes from one infected pool into another can carry it along. A family visiting a pool in Florida with an active bloom and a swimsuit that never actually dried between pools, can seed a pool in Kansas City that had nothing wrong with it the day before.

Black algae is an algae. It's cyanobacteria, one of the oldest organisms on the planet, older than anything else we're talking about today by a couple of billion years. It builds an actual multilayered shield around itself, mostly polysaccharides, and it sends root like structures down into porous surfaces like plaster, gunnite, and grout, anchoring itself in place. That's why brushing alone never works on black algae.

And then there's pink. Pink isn't algae either. It's not even in the same kingdom as the other three. It's a bacteria called seratia marcisens, and it's not feeding on sunlight the way the others are. It's feeding on you. Body oils, sunscreen, lotion, everything that rinses off a swimmer, that's its food source. It builds up in the same low flow corners as the others for the same reason. That's where nobody's chlorine ever really reaches. From a history standpoint, pink algae was a major issue with non-chlorine big guanide pools back in the day, and it was a major contributor to the decline in popularity of that sanitizing system over the last thirty years.

Four organisms, three different survival strategies, and one of them isn't even the thing you thought it was. That's the last piece we're sitting with before we get to why any of this dies at all. All four of these have one thing in common, and it's not a coincidence.

You've heard me say it on this show before. Free chlorine should sit at a minimum of seven and a half percent of your CYA level. I've never told you exactly why that specific number is the one that matters. Cyanuric acid doesn't just sit next to your chlorine in the water. It bonds with it temporarily, in a reversible reaction. Think of CYA as a reservoir. Chlorine checks in, gets released back out as needed, then checks back in again. That's what protects it from the sun. Without CYA, direct sunlight can burn off most of your chlorine in a couple of hours. While chlorine is checked into that reservoir, an important detail almost nobody explains is that it's not doing any killing.

The only form of chlorine that actually does the work, the only form small enough and reactive enough to get through a cell wall, is hypochlorous acid, HOCL. And at any given moment, the vast majority of your chlorine isn't HOCL. It's checked into the CYA reservoir, chemically real, but momentarily inert. At 30 parts per million CYA, something like ninety-seven percent of your chlorine is bound up that way.

You could be testing four parts per million free chlorine and only have a tiny fraction of a percent actually available to do anything in the next second. Now, remember the math from a few minutes ago? Algae doubling, generation after generation, invisible right up until it innocent. That's a race. Your active HOCL has to be killing cells faster than they're splitting. Not on average over the whole day. Right now, continuously in every corner of the pool. 7.5% of CYA in free chlorine is the number that keeps enough HOCL actively circulating to win that race under normal conditions. Fall under it, and you haven't done anything wrong that a test strip would ever show you.

Your CYA reservoir is just holding back more chlorine than your active kill rate can afford. And the algae's doubling clock doesn't care that the number on your test kit still looks reasonable. That's not a chlorine problem. That's a math problem wearing a chlorine costume.

This is the one that breaks people's brains. You test your water. Free chlorine reads fine. Three parts per million, four parts per million, whatever your target is. And the pool is still green. That test isn't lying to you. It's just telling you something true about one moment, not about the whole day. Free chlorine is a supply. Whatever's consuming it, organic waste, sunscreen, a growing algae bloom, is a demand on that supply. Your test measures whatever's left over after that fight at the exact second you dip the strip or filled the vial. It tells you nothing about how fast that number is moving or in which direction.

If you have a salt cell or a tablet feeder working to hit a target number, it'll keep replenishing chlorine to chase that target all day long. So the reading looks healthy, consistently healthy even. What that reading doesn't show you is that an active bloom is consuming chlorine just as fast as the system can produce it. You're not looking at a pool that has enough chlorine. You're looking at a pool locked in a fight, it's barely surviving, and the scoreboard only shows you the score at halftime.

And think back to that CYA reservoir I explained a few minutes ago. Whatever chlorine you're reading, a good chunk of it is checked into that reservoir at any given instant, not actually circulating as active hypochlorous acid. So even a number that looks comfortable on paper might represent a much thinner actual kill rate than it appears to.

Add those two things together, a bloom consuming chlorine in real time, and CYA holding back a chunk of whatever's left, and you get a pool that tests fine and looks like a swamp. Not a broken test, not bad luck. Just a supply and demand race you can't see by looking at the number once.

The fix isn't a different chemical. It's raising supply enough and for long enough that it actually wins the race instead of just keeping pace with it. So what actually happens when chlorine kills an algae cell? Not the marketing version, the real one. It comes down to that molecule called hypochlorous acid HOCL. It's small and it carries no electrical charge, which turns out to matter enormously. Because it's neutral and about the same size as a water molecule, it can slip straight through an algae cell's outer membrane, the same way water does. Most chemicals can't do that. Hypochlorous acid can. Once it's inside the cell, it doesn't just politely ask the cell to stop functioning.

It oxidizes proteins and other cell components directly, and the cell's internal machinery starts failing from the inside. At the same time, HOCL and its close relative, the hypochlorite ion, are attacking from the outside too, breaking down the structural material of the cell wall. It's a two-front assault, inside and out, all at the same time.

Free chlorine and combined chlorine diverge sharply here, and almost nobody explains why. Free chlorine doesn't just damage a cell, it dissolves it. The walls come apart, the internal contents disperse, and the cell essentially stops existing as a distinct object. In controlled studies, free chlorine destroyed close to ninety eight percent of the actual chlorophyll content in the algae it was exposed to. Total collapse. DRT, dead right there.

Combined chlorine, the chloramines, work completely differently. They're bulkier, less reactive molecules. They don't punch through the membrane the same way. What they mostly do is disrupt the cell's ability to manage water and pressure across that membrane, and the cell shrinks. Wilts almost. It dies functionally, but it stays largely intact as a shrunken husk instead of breaking apart. In the same studies, dichloramine only managed to destroy about 64% of chlorophyll content, doing the same job free chlorine did, at a third of the effectiveness. Same element, two completely different outcomes, dissolution versus shrinkage, depending entirely on which form of chlorine is doing the attacking. That's the real mechanical reason free chlorine is the number that matters on your test kit, not total chlorine.

Combined chlorine can be sitting there, technically present, and mostly just making your algae smaller instead of making it gone. Now, this whole mechanism assumes hypochlorous acid can actually reach the cell in the first place. That's exactly where black algae shield and mustard algae's coating come back into the picture. We'll get to that, but first, the kill isn't actually the end of the story. It's the beginning of a different one. Killing the bloom isn't the end of the job. It's the halfway point. Remember what we just covered. Free chlorine doesn't just kill a cell, it dissolves it. That's why it's more effective. It's also why a properly killed bloom can leave your water cloudier than it was before you started treating it.

You're not looking at failure. You're looking at a few trillion cells breaking apart into fine suspended debris all at once, hanging in the water because it's too small to settle and too fine for a struggling filter to catch efficiently. That cloudiness is actually a decent sign that the kill is working. It's just not the finish line. An important part of this gets skipped constantly. When an algae cell dies and breaks apart, it doesn't just vanish. Everything that was inside it releases back into the water. Organic matter, and phosphorus in the form of phosphates. Phosphates are food, specifically their algae food. A bloom you just spent a day killing can leave behind exactly the nutrient load a fresh bloom needs to get started.

This is the actual mechanism behind the pool owner's worst nightmare treating an algae problem, watching it clear, and having it come roaring back within a week. The treatment didn't fail. Its own casualties fed the next generation. So the real protocol isn't kill and walk away. It's kill, then physically remove the debris fill the kill just created. Vacuum to waste where you can so that the fine material leaves the pool entirely instead of just cycling through, back into the water on the return side. Clean or backwash the filter because a filter loaded with the less broom's debris has no capacity left to catch anything from this one.

Skip that second half, and the problem hasn't actually been solved. It's just been fed, cleared enough to feel finished, with the timer already running on the next one. So everything up to this point has been the why. This is the how with real numbers for all four organisms using nothing but chlorine.

Remember the 7.5% number from earlier. That's the prevention minimum, the least amount of active free chlorine that keeps a bloom from ever establishing. Killing an established bloom that's already doubling on its own clock takes a different, higher minimum, and it's built on exactly the same map. Raise free chlorine to at least 40% of your CYA level, not seven and a half. A pool sitting at 30 parts per million CYA needs free chlorine of at least around 12 parts per million to actively win that fight. At 50 parts per million CYA, that's at least 20 parts per million. Same ratio-based logic as before, just a different floor for a different job.

Prevention versus active combat. Remember the reactor from a while ago? Normal operation holds the multiplication factor right around one. Steady, controlled. 7.5% of CYA is that steady state for your pool. But when a reactor needs the reaction stopped immediately, not managed, but stopped, you scram the rods. Full absorption all at once, overwhelming force instead of gradual correction. Forty percent of your CYA is your scram. It's not a bigger dose of the same maintenance move. It's a completely different move on purpose because an active bloom isn't a maintenance problem anymore.

Use plain, unstabilized liquid chlorine for this, not dichlor or trichlor. Those add more CYA with every dose, and raising CYA mid-fight raises the very minimum you're trying to clear, working against yourself in real time. And this isn't a single dump. It's a level you have to maintain, testing and redosing to hold it, not a number you hit once and walk away from.

Green clears the fastest. It floats free and carries no real defenses, so correct free chlorine relative to CYA is genuinely sufficient on its own. Hold that minimum level, keep the pump running continuously, and correct free chlorine does the entire job. No brushing required because there's nothing really protecting the cell. Mustard is a different story, and it can be a pain to kill. It clings to surfaces instead of floating, and it evolved a protective coating, standard maintenance chlorine was never going to punch through. That coating has to get physically disrupted for the chlorine to actually make contact. So brushing happens alongside. The chlorine, not instead of it.

Brush first, then let the elevated chlorine level do its work. Then brush again while it's still elevated. On the number itself, 40% of CYA is the floor, but in the field, mustard has consistently needed more than that floor to actually clear. Free chlorine at 50 parts per million or above, sustained for two to five days, is the figure that has reliably worked across real pools, not just the ratio math. Treat 40% of CYA as the minimum you never go under, and 50 parts per million sustained as the number that actually finishes the job. Black takes real persistence. That multilayered shield we described isn't cosmetic. It's a physical barrier standing between hypochlorous acid and the cell it's trying to reach.

No amount of free chlorine matters if it never makes contact. Aggressive brushing, ideally with a stiff or stainless brush, depending on the surface, has to tear through that shield first, repeated, because the shield can partially regenerate. Brush, hold the elevated level, brush again, retest, and repeat the cycle until the shield stops coming back.

Pink responds to the same oxidative mechanism as the others, since it's dying to the exact dissolution process we covered a while ago. What changes is the other lever available. Because it's feeding on body oils and sunscreen rather than sunlight, cutting its food supply, showering before swimming, actually helps the chlorine keep up in a way that doesn't apply to the other three. And notice where all four of these actually take hold Corners, behind ladders, wherever the water barely moves. That's not a coincidence. Four different organisms, four different survival strategies, and every single one of them is chasing the same thing. Water that never quite moves.

One tool, used differently depending on what's actually standing between it and the cell. No bottle of algecide anywhere in that description. Chlorine handles the kill. It was never built to handle everything that happens around the kill, and that's exactly where a couple of real, narrowly targeted products earn their place. The same two I actually run across the field laboratory every week on hundreds of pools. Remember the phosphate problem from a bit ago? A dead cell releases what was inside it, including phosphates, straight back into the water, feeding whatever tries to grow next. Chlorine can't touch that. Phosphates and chlorine don't react with each other directly, so no amount of free chlorine, correctly dosed or otherwise, does anything to remove them.

That's the actual job of a phosphate remover, and the one I use is Orenda's PR ten thousand. It's a concentrated, rare earth-based compound that converts dissolved phosphate into a solid, precipitating it out of the water so it can be vacuumed or filtered away instead of sitting there as food for the next generation. Applied right after a shock, it closes the exact loop we open in the after death part. It doesn't kill anything. It just makes sure the last kills leftovers can't start the next one. It's on the truck for that reason, not because a label told me to carry it. The other real gap is bather waste.

Body oils, lotion, sunscreen, everything that rinses off a swimmer. It doesn't just feed pink slime specifically. It's organic material and left alone. Chlorine has to burn itself, oxidizing it, the same way it burns itself fighting an active bloom. Every bit of that load is chlorine that isn't available for the actual fight. The enzyme product I run is Arenda's CV seven hundred. It digests that non-living organic material directly, breaking it down through its own catalytic process rather than asking chlorine to do it. Less demand on the supply side means more of your free chlorine stays available for the job only chlorine can do. Neither of these is standing in for chlorine, and neither is pretending to be a kill agent wearing the wrong name.

One removes a nutrient chlorine physically cannot touch. The other removes a demand chlorine would otherwise have to absorb. Different problems, real solutions, no overselling required. It's worth being precise about what neither of these does. Phosphate removal does not prevent a bloom on its own, and it's not a substitute for correct free chlorine relative to CYA, which is still the actual defense. It is a targeted cleanup step for a specific mechanism, not a stand-in for the chemistry that does the real work.

There is a piece of this that matters even in a pool that never visibly blooms. Correct chlorine is not just standing by for the day something goes green. It's constantly working against the same dormant, low level population we opened this episode with. The algae that is always present, always waiting. That is baseline chlorine demand, and it means a small amount of dissolution and a small amount of phosphate release is happening continuously in every pool, blooming or not. That's the real case for PR 10,000 as a standing practice on the route. Not insurance, a proactive buffer. Every sanitizer system fails eventually. A chlorinator malfunction, assault cell dies, a floater runs empty over a weekend because nobody's watching.

When that happens, what matters is how much phosphate was already sitting in the water, waiting for the moment chlorine coverage lapsed. A pool kept clear of that standing reserve of phosphates has real room before a gap in coverage turns into a bloom. A pool that has been quietly accumulating it does not. CV seven runs on the same logic from the other direction, keeping the bather waste from becoming chlorine demand that was never really about killing algae in the first place.

So let's talk about algae. Walk into any pool store aisle and you'll find three real families of algacide quats, polyquats, and copper. They don't work the same way and they don't fail the same way either. Quats, short for quaternary ammonium compounds, carry a positive charge. Algae cell walls carry a negative charge. Opposite charges attract, so the quad molecule latches onto the outside of the cell and acts almost like a detergent, breaking down the cell's outer structure until it leaks and dies. It's a real kill mechanism. The trade-off is foam. Quats are surfactants, the same family of chemistry behind dish soap, and dose them too generously, and your pool grows ahead of foam like a beer.

Polyquats work on the same basic principle positive charge, negative charge, membrane disruption, but the molecule is a long chain instead of a single unit. That longer chain sticks to surfaces better and lasts longer in the water, which is why it's the non-foaming, more expensive option on the shelf. It's also degraded over time by the very chlorine sitting next to it in the water, so its residual effect has a real expiration date, not an indefinite one. Copper-based algacides are a different animal entirely. They don't attack the cell wall at all. Copper ions interfere directly with photosynthesis, disrupting the exact biochemical machinery that algae used to turn sunlight into energy.

Cut off that machinery, and the organism can't sustain itself. That's also the answer to something worth sitting with for a second. Copper does essentially nothing to bacteria like the pink slime we talked about earlier, because bacteria aren't running a photosynthetic engine that copper can jam. Copper's entire weapon only works against something that photosynthesizes in the first place, which means it's aimed at three of our four organisms and structurally useless against the fourth. That's the real menu. A detergent that foams, a longer version of the same detergent that chlorine slowly eats, and a metal that only works on organisms running on sunlight. All three are real chemistry doing real work.

They're also a lot more specific and a lot more limited than algicide as a category name lets on. And copper alga didn't start in a swimming pool. It started in a vineyard. Back in the eighteen eighties, French grape growers started mixing copper sulfate with lime and spraying it on vines to fight downy mildew. It worked well enough that it's still used in agriculture today. It's called the Bordeaux mixture. Copper's ability to disrupt a plant like organism's biology isn't new science. It's roughly 140 years old. When backyard pools started showing up at scale in the middle of the twentieth century, copper sulfate was already sitting there, cheap, shelf stable, well understood, and it made green water visibly clear within a day or two.

That's a powerful thing to witness as a pool owner with no test kit and no real chemistry education. You dump in some blue crystals, and by the next afternoon, the water's blue again instead of green. Of course, that became the story people told each other. What that story usually leaves out is that a lot of those pools were also getting a real chlorine dose at the same time, because nobody treats an algae problem with one product and nothing else. The copper took the credit. The actual oxidation happening alongside it did the harder, less visible part of the work. Add decades of that pattern repeating, a visibly fast product sold next to a slower, less dramatic chemistry fix, and what you get is exactly what's still on the shelf today.

A product that took the credit for a job the water's own chemistry mostly finished. Now that you know what's actually in the bottle of algicide, and now you know how it's earned the spot on the shelf, what matters is what you do with both of those facts. Against a bloom that's actively doubling on the clock we described earlier, a detergent style membrane disruptor or a photosynthesis blocker is playing a slower, gentler game than an oxidizer that dissolves a cell outright. It's not that these tools do nothing, it's that they can't win a race against exponential growth the way hypochlorous acid can. The name algicide itself undersells what these products actually are.

As I've said before, algicide implies a primary kill agent, something built to end a live infestation. What quat, polyquat, and copper are mostly suited for is a retardant and a preventative. Something that slows colonization and adds residual protection between treatments. It's a legitimate job, it's just a different job than the one that the name advertises, and a different job than the one decades of habit trained people to reach for it to do. Correct free chlorine relative to CYA, the math from earlier in this episode, is almost always sufficient on its own to prevent a bloom from ever establishing in the first place. That's the actual reason algicide isn't standard practice on any of our service vehicles for ordinary algae management.

Simply because the chemistry that's already supposed to be in that pool makes it redundant. There's one situation where that changes, and it's narrow enough to name specifically. During a documented zero free chlorine treatment window, the kind used in certain metal staining protocols, there's no active HOCL in the water at all to rely on. In that one specific case, and only for the duration of that window, a nonmetallic polyquat formula is the correct tool, because for those few days, it's genuinely the only chemistry available to do the job. Everywhere else, the bottle of algecide isn't the answer. The math from twenty minutes ago already was.

So let's go back to where we started. A quiet corner, a little warmth, a gap between what the corner needed and what it actually had. Everything we covered today, the doubling math, the CYA reservoir, the four species, the actual mechanics of a kill, what actually belongs next to chlorine, what algicide really is and isn't, all of it eventually leads back to that same corner. Species after species, decade after decade of pool owners fighting this, and the pattern never changes. It's never the whole pool, it's the one place the water forgot to visit. That's worth sitting with because it means the fight was never purely a chemistry fight.

Chemistry wins the battle once it actually arrives. Getting it there in the first place is a different question entirely, and an important one we haven't touched yet. Hydraulics, fluid flows, thermodynamics, the physics behind the chemistry. Every study and figure in this episode, the chlorophyll numbers, the CYA chemistry, all of it, is cited in the full show notes at poolsscientific.com if you want to go check my work.

Here's something to think about next time you're standing at your equipment pad. It's not just what's in the water, it's where the water actually goes, and where it never quite reaches. We'll dive into that next week. The Pools Scientific Podcast. Visit us at poolsscientific.com