The Dark Side of Pools: Black Algae Survival Tactics

Black algae has a way of making even well‑kept pools look defeated. We dig into what it really is—cyanobacteria protected by a stubborn biofilm—why it laughs at normal chlorine levels, and how it burrows into plaster and Pebble Tec where a quick brush won’t touch the root. If you’ve spotted dime‑size black dots that keep coming back, this is your field guide to taking control without trashing your finish.

We share a step‑by‑step plan that starts with breaking the biofilm using the right tools: a small stainless steel brush for plaster and Pebble Tec, nylon on fiberglass, plus a stain eraser for tight spots. Then we layer in chemistry the smart way. Chelated copper (like PoolRx) provides steady background pressure, while targeted oxidizers finish the job. You’ll learn when granular trichlor is the fastest hammer on plaster, why it can burn colored finishes, and how to use calcium hypochlorite as a safer alternative—especially in cooler water where it lingers on the spot. We explain the tablet‑chunk method for cal hypo, why dichlor dissolves too fast, and how to broadcast without clumping to avoid stains.

For extreme infestations, we outline when a drain and chlorine wash beats an acid wash, the real‑world risks to aging plaster, and how commercial pools benefit from a full reset that also addresses high cyanuric acid. Prevention gets equal airtime: keep free chlorine from crashing to zero, hold 5 ppm or more during recovery, maintain chelated copper to deter regrowth, and stop cross‑contamination by dedicating brushes and nets to infected pools. We also highlight the early signs pros catch—pencil‑eraser dots on steps and crevices—so you can act before colonies spread.

• black algae as cyanobacteria with protective biofilm
• why worn plaster and crevices worsen colonization
• cross‑contamination from shared tools and how to avoid it
• copper as steady background pressure and limits
• trichlor granular for plaster and risks on colored finishes
• cal hypo as safer Pebble Tec option, tablet chunk method
• when to shock to 30–50 ppm and why
• extre

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Transcript

Welcome And Topic Setup

SPEAKER_00 1:02

And welcome to the Pool Guy Podcast Show. In this episode, I'm going to talk to you about black algae. I'll go over some treatment methods as well as some cautions with using the same tools in the pool with black algae. And I'll discover just about everything regarding black algae in this podcast for you. Are you a pool service pro looking to take your business to the next level? Join the Pool Guy Coaching Program. Get expert advice, business tips, exclusive content, and get direct support from me. I'm a 35-year veteran in the industry. Whether you're starting out or scaling up, I've got the tools to help you succeed. Learn more at swimmingpoollearning.com. So black algae is different than your standard algae. In fact, it's more like the plague when it's in the pool, and it's something that is more or less a big problem and a nightmare out there on your pool route if you get pools of black algae. The treatment, of course, is intensive, and there's a lot of different methods you can use, and I'll cover those a little bit later. Let me just explain what black algae is exactly. It's technically not an algae, it's more or less a it has a it's a biofilm that's resistant to the chlorine that's in the pool, first of all. So you're gonna find that even with a good chlorine level, the black algae still persists on the surface. It's actually caused by a cyanobacteria, and it's been called an algae for years in the industry because you know it's just one of those things where the research wasn't there back then, and it's gonna be called an algae probably for a very long time. And black algae and it in the name itself has the kind of misnomer of what it is, and this biofilm actually is really something that makes it hard to get rid of. The protective layer is something that protects the the cells of the black algae, the I should call it cyanobacteria, but I'm kind of stuck with the black algae moniker. Black algae, it embeds in the plaster, and so you'll notice that you can brush off the head of the black algae, but you'll still see part of it inside the plaster itself. Really, when the plaster is wearing out and cracking, you'll see a lot more black algae forming in the pool, and it's something that makes it really chemical resistant, this biofilm on top. And that's why it's important during your treatment to brush the heads off with a stainless steel brush or to treat it with a topical granular trichlor type uh chlorine that will take the biofilm top off of it as well. I would caution about using the same tools like a brush, a leaf rake in a pool with black algae. The question comes up, you know, does black algae spread from pool to pool on your tools that you use in the pool? And it does spread from pool to pool, in my opinion. It's you know, how does it get in the pool in the first place? That's something that is a question you may ask. The old, I guess, myth was that it came from the ocean when people were in the ocean on their bathing suits, the black algae would get into the bathing suit, they would go into a pool and would get in there, but I don't really think that holds up as well as of course using a brush and one pool of black algae, and then the pool that you're using and it gets black algae. That's much more logical, I think, and something that I've seen happen. So, yes, using those tools in one pool to another, I would suggest just leaving your stainless steel brush and a leaf rake at that black algae pool so that when you go back there, you're gonna be using that specific tool set to clean that pool, and you're not gonna take it to your other pools. Now, black algae again is very resistant to chlorine, and this is why you'll sometimes see it spreading in a pool, even if you keep the chlorine level elevated in that pool. So, let me go over some treatment methods here. I think it's important to cover these here, and you just can't brush them with a stainless steel brush and call it a day because you're just spreading that back black algae throughout the pool at that point. What you need to do is do a pretty aggressive treatment with either copper in the water. Now, the Pool RX is a great product for this because it has copper as one of the minerals in there, and copper is actually known to destroy the cyanobacteria. Now it's a slow process where it may take weeks with the copper elevated in the pool to actually affect the black algae. And it also depends on how much black algae is actually in the pool. From my field experience, the more black algae in the pool, the less likely the Polar X copper or copper algae side is going to have a dramatic effect on it. It will, of course, reduce the black algae, but if the whole deep end is covered in black algae, and typically the black algae are circular in appearance, they're usually the size of a penny, sometimes up to a quarter. Sometimes they can join together and form bigger splotches, but typically they're circular and they're spread out along the pool, along the walls of the pool, wherever it can get inside the plaster, pebble tech, it'll form into those crevices, and that's why the combination approach is the best. So, number one, a copper algebracyte or the pull axe is a great way to start. So start with that, and then you're going to want to brush them off the sides of the wall with a stainless steel brush. Now, caution if you have black algae in a fiberglass pool, you really don't need a stainless steel brush. And yes, you can get black algae in a fiberglass pool. I've had it in a fiberglass pool before. You can use a regular nylon brush, maybe a small one, and brush it aggressively. Because if you use a stainless steel brush, you may damage the gel coat on the fiberglass pool. And Pebble Tech, you can use a stainless steel brush, no problem. Now, as far as the second part of the treatment, here's where it gets kind of tricky, and you have to be really cautious with this secondary treatment, and that's using granular trichlor. Hasa makes an algeban or a product called Algeban, which is a granular trichlor. And the caution is this this can burn the surface of the pool, it can actually stain any painted, not painted, I shouldn't say that, any colored pool, I guess painted as well, and it could even stain or burn the pebble tech surface as well. It'll definitely burn the fiberglass surface and it'll stain the vinyl surface. You don't really get black algae in vinyl pools, but you don't want to use trichlor granular in a fiberglass or vinyl pool because it will stain that surface. Now, using it in a plaster pool is perfectly acceptable. Typically, the black algae is in older pools, and usually the plaster has stains in it already, it doesn't look that great. Newer pools, yes, you can get black algae as well. So use that trichlor granular with caution. What I really like using in the Pebble Tech pools, and I like treating the black algae before summer or usually when it's cooler, and this is more effective in colder water, is using Calhypo, the just pouring the Calhypo directly over the pool surface, maybe using you know six or seven pounds of it, and then sprinkling it over the black algae in the pebble tech pool. I don't really like using the trichlore in a pebble tech pool because it can burn that pebble tech surface. Usually it's a tan color pebble tech or a darker color, and the trichlor doesn't go well with that. So the cal hypo works really well as a substitute chlorine that sits on top of the black algae long enough. If you leave the pool off, of course, for a couple hours, turn the cleaner off, and let that cal hypo just sit on the pebbletech surface. The pool's gonna get cloudy, of course, but it does have a pretty good effect after you brush the heads off with the stainless steel brush, of killing that black algae because the calhypo doesn't dissolve quite as fast in colder water again. In warm water it may dissolve really rapidly. And dichlor isn't really a great choice because that dissolves too fast, in my opinion, to have any effect on the black algae. One thing that I've been experimenting with is breaking up calhypo tablets and using that as a granular form of calhypo, and it's really effective. So if you have a black algae in a pebble tech pool or even a newer plaster pool or plaster pool where you don't want to use the trichlor because you're afraid of it damaging the surface, I would suggest getting some calhypo tablets, breaking them up into chunks, not quite really fine little granulars, but just chunks of calhypo, and then broadcasting that into the pool, letting that sit on the plaster or pebble tech, and it's going to have a really good effect on the black algae or killing the black algae, and it's going to be much safer to use than trichlor. I told the calhypo manufacturers to start making this product. I don't know if they're going to take my advice or not, but instead of just the powdered form, which of course is always faster, if they can make a larger granular form of calhypo, it would be really highly effective in treating black algae in pools without having to worry about staining. Now it will stain a vinyl pool, so don't use calhypo in a vinyl pool, either broadcasting it and letting it sit on the bottom. So just that's another caution for vinyl pools. But it's a great kind of in-between the calhypogranular or powder, I should say, making the tablets into a calhypogranular versus using the trichlor granular, which can burn the pool surface. Now, if you do use a trichlor granular, which I like using because it's highly effective, not only does it raise the chlorine level really high in the pool, but it sits on the black algae. And again, you have to brush the head off again with a stainless steel brush. So this is part of the process. You can't just broadcast the trichlorine the pool and hope for the best because that biofilm is pretty strong and thick, and sometimes it takes a lot to get to the head or the root of the black algae. And brushing with a stainless steel brush, I like using I like using a five-inch stainless steel brush because that really gives you that power to brush those heads off. The 10-inch or 12-inch stainless steel brush is highly ineffective. You need something really small to get in there. I also like using a product called the stain eraser, it attaches to the pool pole, it looks almost like sandpaper kind of coagulated together, and it's highly effective in getting those heads off as well. When you're in an area where the stainless steel brush can't reach, get the stain eraser and kind of brush it off. I wouldn't use a rust stone, so to speak. May scratch the plaster getting the head off, but once you brush the heads off of the black algae, then you get the trichlor and you sprinkle it on to the bottom of the pool, and you want to avoid it clumping up. You want to make sure that it's just broadcast, you know, so that it's not clumped up and piled up in the area because that's where it actually burns the surface when it piles up. And it really doesn't take too much of the algae band to actually work. You just sprinkle it in there and it takes care of it. So it doesn't need to be piled up anyway, it just has to touch the residue of the black algae head after you scrape it off, and that's all you really need to do. Now, in extreme cases, you may have to drain the pool and give it a chlorine wash. This is when there's black algae all over the pool, you know, shallow and deep end on the walls. This calls for a full drain and a chlorine wash. Sometimes you have to actually, and this has happened before, we have to power sand the head of the black algae off, that's how bad it gets. There's like a whole colony of black algae and it's in an area, and you just have to sand it. Now I wouldn't recommend acid washing the pool, it doesn't really do what the chlorine wash will do. A lot of times the surface already has cracks in it and has pits in it, so acid washing it will cause even more damage to the pool. And it's one of those things where it's one of those what's the greater evil? Draining the pool, chlorine washing the pool with the potential of having the plaster crack and chip more. And of course, you need to have a waiver with the customer letting them know that you're draining the pool to chlorine wash it, and you want to make sure that you know they are they're aware that the plaster could be affected by that. For commercial pools, a lot of times you have to drain them, chlorine wash, and because doing the topical treatment is not usually advisable or appropriate, you have to close the pool down for a few days. There's a big mess, and so sometimes you have to drain it anyway to clear out the cyaneric acid. So you might as well drain it at that point, refill it after the chlorine wash, and try to keep the chlorine level at a certain level. At some point, the black algae may return if the chlorine zeroes out. And so, here's another thing that's really important. I would definitely recommend a copper algebracyte continuously in that pool. So the pool RX is great because it has that chielated, so it's not going to drop out and stay in the pool. There's going to be a constant copper being put into the pool water without any issues. And then, secondly, you're going to have the chlorine level elevated and never let it get you know down to zero. Always keep it at five parts a million or above continuously so the black algae has no chance of reforming in the pool. And the copper algae side will also keep it at bay. The real key to preventing it is to have the chlorine level at a good level all the time, and also having that copper algecide at the same time. So those two in conjunction really go a long way in preventing the black algae from returning because your whole goal is to eliminate the black algae with the chemical treatment, and then of course keep it from returning with continuous vigilance with the chlorine level elevated and with the copper algisite or the polar X plus of the copper mineral being in the water continuously, and that should solve the black algae problem going forward. There's really not much to it except that you have to treat it and really get on it before it spreads. If you just ignore the black algae, it's going to eventually spread throughout the whole pool. So once you first start seeing it again, it may just be really small, maybe even smaller than the size of a dime, and it may be the size of a pencil eraser at the beginning. But those grow, get bigger, spread. And so if you see little pencil eraser marks all over the pool, or even smaller ones than that, it's more than likely black algae. Brushing it, treating it with a topical sanitizer and the copper algae side. Now, if you just have a small amount, you know, in the pool, the copper algae should be effective in treating that black algae. You wouldn't need the full chemical, you know, topical treatment that I mentioned here. But if it does start to spread and you do notice more black algae, then yes, the topical chemical will come in handy to treat it. And you you are raising the chlorine level again to about 30 to 50 parts per million. So any black algae on the wall will come off. They do have tablet holders, you know, they make tablet holders, trichlor tablet holders you can use and scrape and leave a residue of the tablet on there. That's somewhat effective on the walls, but just raising the chlorine level sometimes is enough. And you can also use that three-inch tablet scraper on the bottom of the pool and leave a streak of that. I'm not a big fan of those, they don't work super effectively, but they do have those, and the three-inch tablet fits in there, and you can use that to leave streaks on the black algae once you brush them off. And it's a little bit more effective on the walls of the pool, but I find that just sprinkling calhypo or trichlor on the black algae head is more effective. Raising the chlorine up to 30 to 50 parts per million using that cal hypo or the granular trichlor, that gets a lot of the black algae off the walls, and it's a really effective treatment. I do have a video using the algae band. It's an older video. You can watch that and see my treatment method. But that's kind of how I treat the black algae, treat it aggressively, don't let it spread and really knock it out before it gets to a point where it becomes a problem for the pool and it just spreads throughout the whole pool, and that can happen for sure with black algae. Looking for other podcasts, you can find those by going to my website, swimmingprolearning.com, and then on the top banner, the podcast icon, click on that, and there'll be a drop down menu of over 1800 podcasts there as well. And if you're interested in the coaching program, you can learn more at poolguidecoaching.com. Thanks for listening to this podcast. Have a great rest of your week and God bless.