Scale or Corrosion: The Silent Destroyers of Pools

Ever run your hand across pool coping and feel sandpaper? That texture tells a story about water that’s either starving for calcium or dumping it everywhere. We take you inside the Langelier Saturation Index—the tool that predicts whether your water will corrode metal and etch plaster or lay down crunchy scale on tile, heaters, and salt cells—and show you how to control it with confidence.

We start by anchoring the basics: calcium hardness sets the foundation, temperature shifts the baseline, and pH is your steering wheel. With live number walkthroughs, we show how a single quart of acid can flip a balanced pool into corrosive red, and how a high pH can nudge a system into scale even when everything “looks fine.” You’ll learn practical target ranges for plaster, fiberglass, and vinyl; why at least 150 ppm calcium is non-negotiable; and how to use alkalinity as a buffer to keep pH moves predictable. We also unpack TDS and salt’s quiet role in scale formation and why measuring them matters in real-world service.

If you handle startups, you’ll get a clear roadmap: set calcium early, keep alkalinity near 100 ppm, adjust pH in steps, and never add calcium chloride and soda ash or baking soda on the same day. We share field stories—from coral-like scale to rusted heat exchangers—that connect the dots between a drifting LSI and expensive repairs. Finally, we reset expectations: you don’t need a perfect zero. Aim for a safe green zone that rides seasonal changes without tipping into damage.

• why LSI predicts corrosion or scale
• calcium hardness as the baseline for balance
• temperature’s impact on the index through seasons
• using pH as the primary steering control
• safe targets for plaster, vinyl, and fiberglass
• live example calculations with app guidance
• startup priorities and minimum calcium levels
• alkalinity as a buffer and sweet spot near 100 ppm
• the danger of adding calcium and alkali the same day
• TDS and salt effects on scale risk
• field stories of etched plaster and crusted coping
• focus on ranges rather than pe

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Transcript

Why LSI Matters

SPEAKER_00 1:02

And welcome to the Pool Game Podcast Show. I have to talk about the LSI, the Lange Lay Saturation Index. And this index is really something that is important in pool water balancing, and it's also something that you should be incorporating into your pool care. So I'm going to explain more about the LSI and how this actually works in residential and commercial pool care. 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 the Ledger Saturation Index has been around for quite some time. And probably the best app for this is the Arenda app. If you just go to if you just go online to arenda.com, you can download the app for your Android or Apple device. And this is something really great and easy to use. Basically, this saturation index is the one we use for pool service. There's different saturation indexes out there, but this one seems to work the best for pool service because of the it's based off of the calcium carbonate in the water, and this will help you determine if the water is corrosive, if we eat away the metal parts of the equipment, the pool plaster, pool surface, and believe me, I've been to a pool where this has happened, or if it's scale forming, we'll leave deposits and calcium and crusty deposits. And I've been to a pool where this is happening as well, and they both look very similar to each other, but they're actually different things going on. One is corrosion and one is scale forming. And this index will help you prevent either of these extremes in most cases in your pool care. You can look at the index basically as a balance between water chemistry factors that will either pull the calcium out of the surface or deposit it on it. I think that's the easiest way to look at it. You're trying to get a balance between the water chemistry factors that will again pull the calcium out of the surface or deposit the calcium onto the surface. It's as simple as that. And it's really something that is predictable and you can use like a rule of thumb. I know Bob Lowry said that you just give him the numbers and you can calculate the saturation index, and most pool pros after a while can do the same thing because it's really easy once you get the numbers, you'll kind of know the different what's out of range and what's in range. Now there's a lot of other indexes out there. You can look them all up if you wanted to. There's the Reisner Stability Index, RSI. There's the Pacoria Scaling Index, PSI, there's the Hamilton Index, and I actually met the person who created that index many years ago, and his son still runs United Chemical, which is a company that was founded, and the Hamilton Index is was used by them and still is promoted. The calcium carbonate participation potential, the CCPP, the calcite saturation index, the CSI. And it's one of those things where the LSI is kind of like an industry standard. And so even though there's other indexes out there, this one seemed to be the easiest one to translate into pool care and pool service. And over the years it's just been established as the number one index that's used out there. Again, for these for this very important thing, which is a balance between water chemistry factors that either pull calcium out of the surface or deposit it on it. Scale forming or corrosive. So the calcium hardness is the basis of the saturation index. This is the one element that everything works off of. It's kind of like the baseline of the LSI, the calcium hardness. So depending on the calcium hardness of the water area, it's going to depend on the other variables like alkalinity and the pH to balance things out. And it's really something that is basic, that the calcium hardness would be the baseline for this. And it's something where the minimum calcium hardness would be 150 parts per million. And of course, for a plaster pool, you have target ranges of the calcium hardness to work off the saturation. So that is kind of easy to manage. You don't want to have too little calcium in the water. So for plaster pools, you would want to have a target calcium hardness of 350 parts per million, and for fiberglass or other surfaces, about 250 parts per million. And too much calcium and magnesium in water does become oversaturated, and this is what happens when you get the scale forming in your plumbing. I think it would be cool if you would get the Arenda app and then play along with me here as I'm moving some numbers around, and I'll show you why the calcium hardness number is the crucial part. I think temperature would be the second most important factor, so to speak, with the LSI. And let me show you here. So I'm gonna give you these numbers here, and if you have the arendap, you can plug these in. 15,000 gallon pool, the current water temperature is 75 degrees, the pH is at 7.5, the total alkalinity is at 100, the calcium hardness is at 480, the cassineric acid is at 80, and the total dissolved solids or salt is at 800, and this is almost at a zero, it's actually a 0.02, which means that it's in the green zone, and you have no problems. But if I actually were to drop the calcium hardness down to say 230, which is fairly low for a pool, not too low for a fiberglass or vinyl pool, and if I were to drop the water temperature down to 70 degrees, now all the other members are the same: pH 7.5, alkalinity 100, the cyaneric acid 80, TDS at 800, but if I were to drop the water temperature down to 70, and drop the calcium hardness down to 230, you're actually at a negative 0.031, which is of course corrosive to the pool water. Now, if I were to leave all those numbers the same, water temperature 70, it just raised the calcium hardness up to somewhere where it's typically in California, which is in the 500 range or so. I'm gonna give it 550 for calcium hardness. And at 550, you're at 0.04, which is green, which is almost near 0, which is almost perfectly balanced on the LSI, just by moving the calcium hardness number up and down. And of course, if I move the water temperature number up, it's gonna be a little more scale forming with that calcium hardness at that level. So let's say the water temperature is 85 degrees, it still will be 0.15, so it does raise it up a little bit, but it's still in kind of the neutral zone where it's not scale forming yet, almost, but not quite there. At the water temperature, if you're in the high desert and it was at like 90, well, I don't know, 95 degrees, still not quite there because the calcium hardness is is a higher number at that point, and it's one of those things where you can see right away with the calculator in your hand that the calcium hardness number is really the crucial one. Now, when it goes to moving the LSI from one range to another, you have these bases, you have the temperature and you have the calcium hardness, which are kind of the base of the LSI, and then you have what would move it from one direction to another, which would be the pH of the pool. The pH is something that's going to move the LSI from scale forming the corrosive in a fast, and of course it affects the alkalinity as well when you lower the pH. So this is kind of what changes the LSI from corrosive to scale forming, is the pH of the pool with the factor of the water temperature and the calcium hardness now, and working with those two baseline items on the LSI, the pH in the pool is what affects the LSI. On if you are going to make adjustments with the LSI, that's just the number you're going to adjust. You can't necessarily adjust the pool temperature. If it's at 70, you're not going to turn the heater on to adjust the LSI, it doesn't make a lot of sense. And you're not going to add calcium hardness to the pool at that point to change it. You would use pH to make the changes in the LSI index at that pool if you find that it's out of range. Changing the pH one way or the other is how you correct that. So let's go back to some original numbers I had in the calculator. Water temperature 75, pH 7.5, alkalinity 100, calcium hardness 550, cyaneric acid 80, and I'm not gonna have the salt pool, I'm gonna bring it back down to 800 for the T DS salt. And so currently we're at 0.08, which is really good on the LSI scale. Out of the app, it has color, so I'm in the green here. And let's just say I put a quart of meritic acid in this 15,000 gallon pool and I drop the pH down to 7. At that point, the LSI goes red to negative 0.38. So there was no need to add acid to the pool at that point because it was actually within range. However, if the pH was at 8 in that pool when I get there and I do the LSI calculation, and again, these numbers are pretty logical. If you figure that if I dropped it to 7, it would cause it to be corrosive. If the pH is at 8 with the other parameters where they're at, you would think that it would be scale forming, and you would be correct. You would be at in the purple, which is 0.54, which is scale forming. So in this case, I would need to add some pH down, which is muritic acid, which is what I use normally. You can use dry acid as well. But if you use myitic acid, you would add a quart to the pool and you would probably drop it down to 7.6, which is in the green range at that point. So it's really important because if you get to that pool and you don't know the calcium hardness, you don't know the alkalinity and all the other factors, which the alkalinity does affect it as well, but not nothing as much as the pH in the pool. You're just kind of blind. And if you have the pH at 8 and you don't know the calcium hardness or the alkalinity again, or even the water temperature, you really can't tell what you're doing to the pool, what kind of effect it would have on the pool, unless you have those other factors. To get a to get the LSI reading and to see if the pool is indeed scale forming or corrosive, the water I should say in the pool. So let me just briefly recap here so you're still tracking with me. The calcium hardness is what I consider the base of the LSI. Of course, that's the calculation is off of the calcium hardness. And then the water temperature is a factor that needs to be accounted for in the LSI. And then the other numbers, pH will change the LSI rapidly in the pool. And the other numbers need to be accounted for, but they don't affect the LSI nearly as much as adding pH to the adding acid or base to the water to change the pH in the water. It's important to note again that this index is based on the calcium saturation in the water. So you're gonna have to have a minimum calcium hardness of 150 parts per million. That's why starting up a pool with a pool startup, it's important that the calcium level is at a certain level for this to be effective and for the pool startup to work effectively. Sometimes the tap water has really low calcium in it. We don't have that problem in Southern California. Our tap water has a lot of calcium hardness in it, but in other areas you may have to add some calcium chloride to the water to bring the calcium hardness up into the range where the LSI index could be effective. So if you do pool startups, the calcium hardness is really a crucial element that you need to focus on to make sure that you can balance the water properly at the beginning. A lot of problems can be offset at the very start, and of course, a lot of problem pools had a bad startup where this was kind of neglected. Then, of course, alkalinity is another factor. I did a podcast recently on pH and adjusting it, and I mentioned that alkalinity is a buffer for pH, and you would want to have it for a startup pool. Of course, you I like to keep it right about 100 parts per million. That seems to be the sweet spot, and that way when the pH is at 7.4, 7.6, there's not much adjustment needed at that point, and it's one of those things where the alkalinity is a big factor in the startup as well. Keeping it within range makes it much easier to balance the LSI as well. There's a warning here I should throw in, and that is you don't want to add calcium chloride to the pool and baking soda, sodium bicarbonate or soda acid, sodium carbonate on the same day. This will immediately cause the calcium carbonate to fall out of the water and it'll deposit all over the pool surface, and it's a real disaster at that point. So if you're going to add calcium chloride to increase the hardness, you don't want to make any kind of alkalinity or pH adjustments that day, and you kind of don't want to add any soda ash or adjust the alkalinity up with baking soda at the same time. Believe me, people do this and it's a real mess, so you don't want to do that and just avoid doing that. But it's really not rocket science, and it is a little complicated, I will say that, because there are a lot of parameters you need to know to get the proper LSI. I think one parameter that people just don't get often is the TDS because they don't have the meter, and it is a factor in the LSI. If you look at the app, you can kind of move that back and forth and get an idea that if you have a saltwater pool with a lot of salt in the water and you have a high pH, you're definitely gonna have more of a scale forming, even with a higher calcium hardness level in the pool. It's all based on how these factors interact with each other. But I do recommend using this because I did mention at the beginning that I've been at pools where the LSI has been out of balance in both directions. So I was at this pool and they wanted me to do service for them. It was actually a relative of one of my customers. I got to the pool and the coping itself just felt like really bad sandpaper. And then I put my hand in the pool and it felt almost like it was like a coral reef feeling because it had so much scale formed on this pool that even the coping had scale on it. It's just how out of balance the LSI was. And then I've been to pools before where it has a similar feel because of the etching of the plaster. I mentioned when I talked about the pH about having equipment where everything was rusted inside the filter, check valve was rusted, the heat exchange was rusted because the water got corrosive, and it's because of just really not balancing the LSI. I mean, there's more to it than that. If the pH gets too low and the alkalinity is low, you're you have more problems than just the LSI. But a lot of this happens when the LSI is corrosive for a very long period of time, and you start to have this effect on it, the corrosive effect on the equipment and the pool surface itself. So it's really crucial that you use the LSI to balance the pool so that you can kind of stay within range at your pools and prevent scale from forming or prevent corrosion of pool equipment and of the pool surface itself. And it's one of those things where is it necessary to have an exact zero? No, it's not necessary to be at exactly zero LSI. There's ranges, and Arenda does a great job with their app in the green range, so you know that yeah, you may be a little bit low heading towards the corrosive end or a little high heading towards the scale end of things, but that's not gonna really affect the water to the degree that you have to worry about it. You really worry about it when it's extremely corrosive or extremely scale forming, in my opinion, and that's when you make the larger adjustments to the LSI. If you're kind of within range, I wouldn't worry too much about it, I wouldn't lose any sleep about it. The pool's perfectly fine, and as you know, the pool water chemistry, the factors are gonna fluctuate week to week over the season. Water temperature again and the colder water is gonna have the acid in the pool, or adding acid to the pool have a greater effect on the LSI than in warmer water. So all these things together work together so you can balance the pool water correctly. And the LSI really isn't that complicated if you get the red app and just play with the numbers. You can kind of see the pattern of what you're doing and learn from that pattern, and you can correct the pools on your pool route once you get those numbers, plug them in. And it's something that's really simple to do, and it's something that's really essential to do again so that you can see if the water is actually corrosive or scale forming. And again, the Arenda app makes it really easy to do the calculation. There's really no excuse not to use their app and get the LSI calculation. Now that you heard this podcast, I think you should have the app already on your phone and you should be using it to do these readings at your