How to Actually Find the Source of Algae in Your Pool

Algae can make you feel like you’re losing your mind: you raise chlorine, the pool looks better, and then the green haze or mustard dust is back next week. We’re breaking that loop by doing something most pool care advice skips, identifying the real source of algae in a swimming pool, not just treating the symptoms. Some pools are simply under heavier “environmental pressure” from pollen, wind, and nearby trees, while others grow algae because of shade, poor circulation, and those stubborn dead spots where sanitizer never truly reaches.

We also get practical about the equipment side of algae prevention. I walk through why a clean, working filter matters so much for chlorine distribution, how seasonal pump runtime often needs a big jump from winter to spring, and why over-backwashing a sand filter can actually hurt performance. If you’ve ever seen low chlorine readings that don’t make sense, worn cartridges or damaged DE grids may be part of the problem.

Then we tackle the chemistry that solves a huge mystery for pool pros and homeowners alike: the free chlorine to cyanuric acid relationship. Using the 7.5% guideline, I explain how a pool can show 10 ppm free chlorine and still grow algae when CYA is very high, especially when trichlor tablets keep pushing stabilizer upward. Finally, we cover prevention add-ons like phosphate remover, enzymes, borates to 50 ppm, and mineral systems like Pool RX, plus the simplest high-impact habit of all: consistent brushing.

If this helps, subscribe, share it with a friend who fights algae, and leave a quick review so more pool owners and service techs can find the show. What’s the one pool on your route that keeps growing algae no matter what you do?

Algae is not inevitable in every pool, so we focus on why certain pools on a route keep getting blooms while others stay clean. We walk through how to pinpoint the real source, then stack prevention tactics that make algae far less likely to return.  
• environmental algae pressure from pollen, trees, and wind-driven debris  
• shaded areas and circulation dead spots where chlorine does not reach  
• filter condition and seasonal runtime as core algae controls  
• sand filter backwashing based on a 10 psi rise  
• the 7.5% free chlorine to cyanuric acid ratio and why “high chlorine” can still fail  
• strategies to avoid runaway CYA from trichlor tablet use  
• phosphate remover and enzymes as algae prevention tools  
• borates to 50 ppm for long-term algistat protection  
• mineral systems like Pool RX to prevent microalgae  
• weekly brushing as a simple, high-impact habit for stubborn pools  

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Transcript

SPEAKER_00 0:14

And welcome to the Pool Green Podcast Show. In this episode, I'm going to talk to you about algae and specifically how to detect or find the source of algae in a pool. And this is kind of important in treatment. It's something that's not talked about very often. So I'm going to unpack this a little deeper here for you today. 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 Sumipoollearning.com. Is algae in a pool inevitable? I would say yes and no. It's not something that is going to affect every pool on your route. It'll

Why Algae Starts In Some Pools

SPEAKER_00 1:02

affect some pools more than others. And some pools will really never develop algae. And the key is to find out why a particular pool is getting algae in the first place. Now a lot of it has to do sometimes with the foliage around the pool. Like in my area, we have a lot of oak trees, and certain times of year they're going to pollinate. You have also p pine trees and you have other trees that have a lot of pollen when they're pollinating or when they're in season. And this leads to an elevated level of algae in pools, of course. If you're in a wind-prone area, chances are you're going to have more algae as well because more stuff will blow into the pool from the surrounding area. Versus an area really with you know cement deck, not too much wind, no trees around it. It's harder for that pool, I think, to develop algae from natural sources than from neglect. So there are two areas I think that you have to focus on when you're thinking about the pools that have algae on your pool route. One area would be just the natural fact that pollen gets in the pool and algae may form in that pool, depending on some other factors, which I'll touch on in a minute. And then you have the maintenance issues, which of course will cause algae as well in the pool. So finding the source sometimes takes a little thought. And some pools, again, are more prone to algae than others. Some pools have more dead areas, so there's a lot to do with circulation as well. You'll find that there are certain pools where you get a little bit of mustard algae, maybe on

Pollen, Wind, Shade, And Dead Spots

SPEAKER_00 2:40

the step area of the pool, or maybe just a spa in the deep end, or sometimes in the spa when it's not circulating too well. And I would say that algae is formed mainly from the dead areas in the pool, areas where the water circulation doesn't really reach, and so it's not getting any kind of measurable chlorine in that area of the pool. It may also be a pool that has a lot of shadow or shaded areas of the pool where algae tends to form. So those are kind of some things that are hard to cure in some pools because you can't fix these dead spots usually. You could try to move return jets here and there, but some pools just have natural dead spots in them where algae will form in the pool. Now there are things you can do to prevent the algae from forming in the pool in the first place, and these are kind of preventative things for all the pools that are helpful. Having a clean filter at the beginning of the season to me is number one. I clean my filters in February and March. That way I know that April, May, June, July, August, part of September, it's going to have a clean filter, and this really helps eliminate a lot of the algae, especially if you clean them before the springtime, where a lot of the natural algae gets into the pools or in areas. This really helps a lot. Something else you can do is also make sure that there's enough runtime at the pools. Going from winter to spring to summer, sometimes you have to dramatically increase the runtime and circulation or flow of the pool to make sure that the algae is not forming in the pool. So these are factors that are definitely in your control, cleaning the filter, increasing the runtime of the pool to make

Filters And Runtime That Prevent Blooms

SPEAKER_00 4:17

sure that, of course, there's plenty of water circulating in the pool. And circulation does two things, of course. It moves the water around, that's kind of important, but it also disperses the chlorine in the pool, and that's also very important. I don't know if you've had a D filter on your pool route that had a torn grid, or maybe you have a had a cartridge filter that was really old on your pool route, and you'll notice that the water is, I guess, circulating to some extent, but the chlorine is not being dispersed properly in the pool, and you'll notice that the chlorine level is really low in the pool. It's not lack of chlorine in a lot of cases because it's mainly lack of the pool circulating and dispensing the chlorine properly in the pool. So that is a factor as well. So a clean filter, a functioning filter, doesn't have tears in the grids, newer cartridges. Now there's a rule that I kind of use out there. I change all the D grids in the D filter every three years. I change the cartridge filters if there's a quad cartridge type filter every three years. There's a couple exceptions. The stay right system three filters, I usually go five to seven years with those cartridges. And for the quad D filters, I also go five to seven years with the quad cartridge filters. But for standard D filters and the four quad type cartridge filter, I change those every three years, and that really helps prevent algae because it really ensures that the pool is going to have really good elements going forward and is something that is a factor with algae. Now, also if you have a sand filter, you only want to backwash the sand filter when the psi goes from the clean psi, which is to say 20, and it goes up by 10 psi to 30. Then you backwash the sand filter. Over backwashing the sand filter actually has the opposite effect and makes it less efficient. So you only want to backwash your sand filter when the PSI rises by 10 over the clean psi, and this will ensure proper circulation and disbursement of chlorine. Now there's a big factor that I haven't touched on yet, which is an important factor, and this is you'll you'll see this in Bob Lowry's material, and I'm excited that Haas is going to have a video series on Bob Lowry's material coming out very soon on their website. And this is something that he talks about, talked about a lot when he did the podcast recordings with me. Talks about a lot in his material, in his books, and this is the ratio that he discovered, and it's a complicated mathematical thing that he's just kind of boiled down for us or made it simple for us, and that is that the free chlorine should be 7.5% of the cyaneric acid level in the pool. We kind of know that the cyaneric acid affects the effectiveness of chlorine more than the pH level in pools, and so Bob Lowry did a lot of research

The 7.5% Chlorine To CYA Rule

SPEAKER_00 7:12

into the exact relationship between cyaneric acid, particularly high cyaneric acid, and the free chlorine level in the pool. We all have had pools out there where the chlorine level is blazing, but there's still algae in the pool, and we're kind of scratching our head trying to figure out why is there algae every week when the chlorine level is at 10 parts per million. This shouldn't be. And you've if you even if you have good circulation, clean a clean filter, the algae can still develop in that. And yes, there is some chlorine-resistant algae, but there's another factor, and that is that your free chlorine level just isn't high enough for the pool to combat the algae with the chlorine level you have in there. Let me give you a practical example, and this will explain why a pool with 10 parts per million would not have enough chlorine based on this factor. Let's just say that the cyaneric acid level, you tested it, and it's showing over 100 parts per million. And then you do a dilution test where you put bottle water and pool water, and then you do the the uh cyaneric acid test, and it's showing still 100, which means that the chlorine level is probably above 200 parts per million at that point. So let's just say it's at 200 parts per million, and so you take 200 and you tighten it by 7.5%, and you're going to have to have a free chlorine level in the in this particular pool with the cyaneric acid level at 200 parts per million at 15 parts per million, which is not hard to do if you're using trichlor tablets, but then of course you're kind of adding to the problem because you're raising the cyaneric acid level as you're adding more tablets, and that's why sometimes when you begin the season you're using two tablets in the pool, then you go to three trichlor tablets, and by the end of the season you're using four trichlor tablets a week because of this fact that you have to continuously raise the free chlorine level up to higher and higher levels to combat the ineffectiveness of the chlorine based on the higher cyaneric acid level. So if this pool has 10 parts per million of chlorine every day, but the cyaneric acid is at 200 parts per million, then you actually would need to be at 15 parts per million for the chlorine to be effective against the algae in the pool. So I would say this is a big factor for sure in kind of solving the mystery of why a certain pool gets a lot of algae in it. But if it's not an elevated cyaneric acid level, it could be other factors. I already touched on the filtration and the circulation as being contributing factors to this. Also, dead spots in the pool are a big contributing factor where there's really not much circulation going on in those areas, and algae will develop. Now, there are some proactive things you can do to make algae development slower, and also to maybe make the formation of algae even not possible in certain pools that have had algae problems in the past, and that is using what I've classified as a chlorine enhancer. There are several of these that work highly effectively. I've I did a season one time out there where I was just using enzymes and phosphate removers exclusively to kind of test them to see how effective they were in preventing algae and pools, and about 20 pools that would regularly get algae. I bought a bottle of phosphate and enzyme, left it at the pool, and put the maintenance dose in there every week, and it was highly effective in preventing algae in these pools that

Phosphates, Borates, And Mineral Systems

SPEAKER_00 10:43

were pretty prone to algae. So phosphate and enzyme, the phosphate and enzyme, I should say, a phosphate remover and enzyme treatment is effective, and the phosphate remover is really good because it removes one of the food sources for algae, thereby making it much harder for the algae to actually grow and bloom in the pool. Another great thing you can do to prevent algae is to add borates to your pools to 50 parts per million. Now, no one really carries the borate product here locally, so I get my borates from or my boric acid powder from doodadesel.com. They'll FedEx it to your house. There's a little bit of a shipping charge, but for a 45-pound bucket, it's actually pretty affordable. And if you bring that pool to 50 parts per million, you can essentially eliminate algae growth in there. It also is a pH buffer and it gives the water a little bit of a shine to it. How do you know how much borese to add to the pool? There's a calculator called the poolcalculator.com. It has a borate calculation formula in there. You can use that. And then you can buy borate test strips to test the borate level in the pools to maintain that 50 parts per million. Now boris don't evaporate out of the pool. It's a lot like a lot like cyaneric acid where it stays in the pool. And so you don't have to add, you know, the 50 parts per million the next season. You just need to add a maintenance dose going forward. And it's a natural algostat, which means it'll prevent the algae from forming in the pool or growing in the pool. It's a great preventative way to prevent algae. The other thing you can use is a mineral system like Pool RX. This is a great way to keep the algae from forming. It also eliminates microalgae and it makes the chlorine much more effective. And this has some metals in it, copper, and it also has a couple other metals in there that will pretty much eliminate or prevent algae from forming in the pool because of the minerals that are introduced into the water. And the minerals are chelated, which means they're suspended and won't fall out of solution and stain the pool, as long as there's no metals present in the water prior to putting the polar ex unit in the pool itself. These are great ways to prevent algae from even forming in the pool. So that's a lot of information there, of course, and kind of finding the source of algae and some things that will help prevent it. I think one of the most preventative things you can do, and this is something that seems very basic, but it is something that can prevent algae in pools, regardless if you use the enhancers. And the thing is, brushing the pool walls is highly effective. Brushing the pool every week when you're at your service account, brushing the steps, brushing the spa, brushing all the walls down pretty vigorously, because as you know, algae actually forms by sticking on the wall of the pool. And if you brush the pool regularly, which I found, this really reduces the chance of algae forming. If you're a homeowner, you can brush the pool every day, and you'll probably hardly get algae on your walls or in the pool because you're dislodging any kind of microalgae and brushing it up, and then the chlorine will effectively kill it. And brushing the pool is something that's vital and important. And I find that brushing the pool is extremely effective in algae prevention. If you kind of

Brushing Strategy And Route Game Plan

SPEAKER_00 14:10

are not doing the full service at the pool, maybe you're vacuuming and skimming, but you're not thoroughly brushing the pool, you're actually doing yourself a disservice out there because brushing the pool is a great preventative way, a great way to prevent algae from forming and growing on the pool walls and step areas of the pool. So try to isolate those certain pools on your pool route that do develop algae more than other pools, and you're gonna find that this is very common where a handful of pools will get algae every season, and a lot of pools will never get algae or hardly ever develop algae, and just kind of focus and kind of get those pools that do get algae often during the season. Dial those pools in, use the enhancers, brush those pools, and kind of analyze all these factors the cineric acid level, the flow, and the filtration. And you can really go a long way in preventing algae out there and saving yourself a lot of time on your pool route. Looking for other podcasts, you can find this on my website, SonyProLearning.com. On the banner, there's a podcast icon. Click on that, and there'll be a drop down menu with over 1900 podcasts. And if you're interested in the coaching program, you can learn more at poolguidecoaching.com. Thanks for listening to this podcast. Have a rest of your week. God bless.