Metal Staining and the Ecosystem of Metals in Pool Water

A customer called me a few years back. She'd been a customer for several years, and we visited her pool every week. Still do. She called to tell me that her pool had turned purple. Not green, not cloudy. Purple. It stayed that way for the better part of a week, and then it faded on its own and it was gone. Today we're going to talk about the ecosystem of metals that exist in your pool water. And in this episode is at least one case that's about as rare as this show gets. In fact, it's so rare, I doubt any of you have ever heard of it, let alone having experienced it.

There's a lot to get through, so let's go.

Real Chemistry, Real Answers. Every Tuesday.

Fill water is the obvious one, since groundwater picks up dissolved iron and manganese from the rock and soil it moves through on its way to your pipes. Since a lot of drinking water comes from wells and rivers and lakes, this makes a lot of sense. Municipal water carries its own version of this story, since a lot of surface water treatment plants use an oxidizer called potassium permanganate to pull iron and manganese out before it ever reaches your house. If dosing runs high or something in the plant malfunctions, some of that unreacted permanganate can slip through, and it's intensely colored even in tiny concentrations. Real chemistry that's happened to more than one municipal system.

Then there's your own equipment. Certain fittings and components contain manganese or copper as part of the alloy, and aggressive, low pH and negative LSI water will corrode that hardware over time and release it directly into your pool, with no outside source required. Lawn and garden products belong on this list too, and this one gets missed constantly. Iron-based lawn treatments, the kind used to green up lawn fast, are typically formulated with manganese right alongside the iron. When a few granules miss the lawn or an overspray drifts on a windy day, they put a real source of metals just a few feet from the water. And then there's contaminated salt, which is real and does happen, but not as often as it gets blamed.

Right or wrong, it has become the default explanation that a lot of people reach for the second they see a stain in a salt pool before anything else gets ruled out. It is a real cause, but it's not always your answer.

Iron, manganese, and copper are the three metals that stain a pool, and they don't behave the same way, so no single home remedy covers all three. Iron oxidizes into a reddish brown, that classic rust stain, and it's the easiest of the three to treat, usually responding to a straightforward ascorbic acid treatment on its own. Manganese is trickier, since it oxidizes into a stain running from brownish black to, under the right conditions, a genuine purple, and plain ascorbic acid alone isn't often enough. It needs a metal sequestrant working alongside it. Copper is its own animal, and depending on what it reacts with, copper staining shows up blue, green, or purple.

We'll spend real time later in this episode on exactly how that purple version forms. Back to that customer with the purple water. It was manganese. Turns out she had a yard care company on a regular schedule, and they were putting down an iron-based lawn treatment to keep her grass green through the summer. Those products carry manganese right alongside the iron, and that treatment was the far more likely explanation than anything happening a mile away at a water treatment plant. Most pool owners have never once thought of their calcium hardness reading as a metal reading, but that's exactly what it is. Calcium is a metal, atomic number 20, right there on the periodic table, not just something that behaves a little bit like one.

Everything I've described so far, iron, manganese, copper, is really the same story wearing different clothes. Something dissolved in your water crossing a threshold and coming out as a solid. Your water holds dissolved calcium in solution right up until your LSI pushes positive, and then that calcium starts precipitating out as calcium carbonate. The same basic move iron and manganese and copper make under their own specific triggers. Magnesium rides along with it in a lot of water sources too, showing up in your hardness number, though it's a minor player compared to calcium. Same principle every time, whether the element is one you'd call a stain or one you'd call scale.

It crosses a line and that line has a name, solubility, the point past which water simply can't hold any more of something in dissolved form. Push a metal or a mineral past its solubility point and it doesn't vanish, it just stops being dissolved and comes out of solution as a solid instead, which is exactly what a stain or a scale deposit actually is. So what actually pushes a dissolved metal past its solubility point? Three real triggers do it, and each one works a different way. The first is oxidation. Your chlorine acts as an oxidizer, stripping an electron away from a dissolved metal ion and pushing it into a higher oxidation state, and that higher state is dramatically less soluble than the original.

This is why a metal that was dissolved in your water for weeks can turn into a visible stain within hours of a shock treatment. The second is a straight pH shift, no oxidation required. Metal hydroxides, the compound of dissolved metal forms once it pairs up with the hydroxide ions in the water, become less and less soluble as pH climbs, so raising your pH alone can push a metal out of solution even if your chlorine never touches it. The third is direct bonding, a specific metal meeting a specific chemical that it has a strong attraction to. The two combine into a brand new compound that was never going to stay dissolved regardless of pH or oxidation state.

That third mechanism has a history a lot older than swimming pools, since iron and tannic acid, the compound that leaches out of decaying leaves, tree bark, and organic debris, bond directly to form a dark, almost black compound. That's not a pool phenomenon I'm describing to you for the first time, even if you've seen it at some point in your own pool. Leaves in the bottom leave a dark stain. In fact, that's literally how ink used to be made. Iron gall ink, the standard writing ink in the Western world for close to a thousand years, is nothing more than iron and tannic acid bonding directly into a stable, deeply colored compound.

Drop a handful of wet leaves into a pool with any dissolved iron sitting in it, and you're watching the same reaction that once filled ink wells. That's what direct bonding actually looks like. Just two specific chemicals finding each other and forming something entirely new. Controlling a dissolved metal before it turns into a stain comes down to two different chemistry tools, sequestration and chelation, and they're not interchangeable even though people use the words like they mean the same thing. A sequestrant works through a single point of electrostatic contact between the sequestering molecule and the metal ion, enough to keep that ion surrounded and suspended in solution, but only one anchor point holding it there.

Push the water hard enough, a strong oxidizer, an aggressive pH swing, and that single point of contact can be overwhelmed, letting the metal ion slip free and precipitate out. Chelation works through something structurally different. Multiple points of contact from the same molecule wrapping around the metal ion from several directions at once and locking into what chemists call a chelate ring. The word itself comes from the Greek chela, meaning claw, the same root behind the word describing a crab or a lobster's pincher. Because that's essentially the geometry involved, several points closing around the ion at the same time rather than one point holding on alone. That multipoint grip is far more stable than a single anchor, which is exactly why chelation, not sequestration, is the chemistry you reach for in an active stain removal process, where the metal has to stay controlled through a genuinely aggressive treatment process rather than just sit quietly suspended in ordinary daily water.

This next case is about as rare as anything I've ever walked into, and I could tell there was a problem on the very first visit to Catherine's pool. In 30 years of managing thousands of pools, I've seen this exact combination twice. I performed a water test. Copper came back at 5.5 parts per million. Cyanuric acid was sitting at 315. Yikes. The LSI had cratered all the way to minus two point six seven. Those three numbers had no business at all showing up in the same pool at the same time. Think about those numbers for a second, because our own ceiling on CYA is 50 parts per million, and she was running at more than six times that.3 to positive 0.3, it's water aggressive enough to actively dissolve things it's touching, which is exactly what it had been doing to her heater for the better part of a year, her entire first season with the new pool.

And the pool floor was covered in purple crystals. So many, in fact, we still call her pool the purple beach. Chlorine tabs drove her CYA up gradually while nobody was watching it. And pH had been running low for the entire time, eating copper straight out of her heater and putting it into solution. That pH was also part of what was dragging her LSI negative, but pH is the actual corrosive agent here, acidic water attacking the metal directly. LSI just told us the water was aggressive overall. It didn't do the corroding by itself. Once enough copper and cyanuric acid were sitting in the same water, the two bonded directly and formed copper cyanurate.

Not a stain, but an actual crystalline compound, precipitating out across the entire floor of the pool. And that 5.5 part per million copper reading only measured what was still dissolved in the water at the moment we tested it. That number alone was already staggering because a normal acceptable amount of copper in a pool sits at around one tenth of one part per million. Catherine's water was running at roughly 55 times what's considered normal, and that reading said nothing about the copper that had already left solution and was sitting on the floor as crystals. Based on the volume of crystal buildup we were looking at, and it was a lot, the real total copper in that system was somewhere likely in the range of 15 to 20 parts per million.

Her entire heat exchanger was in that water. And that's on the order of 150 to 200 times normal, and three to four times what the water test alone would have told you. A dissolved reading taken after precipitation has already started only shows you what's left over once the reaction is done, not the whole problem. We corrected it in three real moves, not three quick ones. First, we vacuumed the crystals out by hand, since at that point they were the physical solids sitting on the pool floor, not a chemistry problem that you can dose your way out of. Her system used a system three cartridge filter, a good one, and those cartridges were far enough gone that cleaning them wasn't going to cut it.

So we replaced them outright rather than risk sending crystal residue right back through the system. Then came the real haul. Because it's a fiberglass pool, we couldn't just drain it and start over, since the full drain risks the shell popping out of the ground once the water no longer counters the surrounding groundwater pressure. Bringing that CYA down meant a partial drain and refill cycle fifteen times. We tested roughly every five cycles, enough to track real progress without chasing a number that hadn't moved much yet. Once the 15th cycle landed the CYA back in a workable range, we rebalanced the water fully and let it settle into normal chemistry from there.

Every salt pool owner has heard some version of this. Salt water is corrosive. You leach your equipment alive, but the actual numbers tell a different story. Ocean water sits around 30,000 to 36,000 parts per million salt. A salt pool runs somewhere around 3,000 to 3600 parts per million, roughly a tenth. So whatever's happening in your backyard, it isn't ocean strength brine, but the science still holds. Two real mechanisms are actually at work though, and they're not the same thing. Chloride ions on their own are largely inert, not oxidizers. Once there's enough of them in the water, they can attack the thin protective layer that gives stainless steel its corrosion resistance.

And once that layer breaks down in one spot, you get a small localized pit rather than an even all-out wear. It's real chemistry, confirmed by people who study corrosion for a living, and it happens well below ocean strength. Every chlorine sanitizer leaves chloride behind once it does its job, not just salt systems. Liquid chlorine does it, tabs do it, and the salt pool just starts with a much bigger reservoir of chloride sitting there from the start. It's not a fundamentally different chemistry, it's just a lot more of the same one. There's a second, separate mechanism. Salt raises your water's conductivity, and when you have different metals in contact with more conductive water, stainless aluminum, brass, copper, you've built yourself a small battery, whether you meant to or not.

A tiny electrical current flows between those metals, and the less noble one in that pairing slowly loses material over time. That's called galvanic corrosion, and it's the real reason a salt pool is more likely than a non-salt pool to eventually show corrosion damage on mixed metal equipment. I didn't learn any of this from a pool chemistry textbook first. I learned it on a submarine, spending years in an environment where saltwater corrosion wasn't a theoretical concern, it was a constant physical maintenance duty. We were polishing brass and bronze fittings routinely because that's exactly the kind of dissimilar metal system I just described, sitting in a genuinely saltwater environment far more aggressive than anything in a backyard pool.

Every pipe system on that boat had sacrificial anodes built in, the same principle we're about to get into for pool equipment, just at a much higher stakes level. And salt rhyme was everywhere. It crusts up everything. Because when salt water evaporates, the salt itself doesn't evaporate with it. It gets left behind, concentrating on every surface the water had touched. Hosing things down with fresh water to keep that rhyme from building up was a constant, ever-present job, not a once-in-a-while task. The bilge pumps never shut off. I bring that up because it's the same chemistry, just turned up to a level most people will never have to deal with.

If a Navy submarine crew is fighting this every single day in an environment built specifically to withstand it, it should tell you something real about why a sacrificial anode belongs on an ordinary pool equipment too, not as an upsell, as actual earned engineering practice. That's exactly the same piece of engineering sitting on a pool pad, just built for a much lower stakes fight. If galvanic corrosion is the problem, a sacrificial zinc anode is about as elegant a piece of chemistry as you'll ever find on a pool pad. Install something called a water bond and wire that zinc fitting into the same bonding grid as your heater, your ladder, your lights, or anything and everything that's metal that's part of your little electrical system.

Since zinc is less noble than nearly everything else sitting in that water, the current eats the zinc first instead of your heater or your ladders or your handrails. It sacrifices itself on purpose, so nothing more expensive has to. It's a low-cost part and it needs periodic replacement, roughly every couple of years, once it's given up enough of itself doing its job. One more thing worth knowing here, and it's good news rather than a caveat. As that zinc anode sacrifices itself, it releases zinc ions directly into your water. But unlike copper or iron, dissolved zinc doesn't cost sustaining. That's exactly why zinc gets chosen for this job in the first place.

It's a metal that can sit in solution and give itself up without creating a new cosmetic problem while it solves the corrosion one. You're not trading one issue for a smaller version of the same issue. You're getting protection with no real downside in the water itself. Think back to the purple beach, that copper cyanurette case. A properly installed anode wouldn't have fixed the pH problem, but it very plausibly would have taken that corrosion hit instead of the heater, buying real time before any copper ever made it into the water at all.

Two more metals belong in this ecosystem, not as contaminants this time, but as legitimate working chemistry. Silver shows up in ionization and mineral-based supplemental sanitizing systems, and it works through something called the oligodynamic effect. Even at trace concentrations, silver ions get inside a microbe's cell membrane, disrupt it directly, and bind to the enzymes that organism needs to breathe and reproduce. Not oxidation, but disabling the microbe from the inside through a completely different mechanism than chlorine. Copper's role against algae is its own precise piece of chemistry, with copper ions specifically interfering with the electron transport chain inside photosystem II, one of the two light capturing complexes an algae cell uses to photosynthesize.

No functioning photosynthesis, no energy, no surviving cell. That's a documented, real mechanism, and it's exactly why copper shows up as a serious tool even against black algae, which isn't algae at all, but a cyanobacterial biofilm that builds its own chemical defenses, one of the toughest organisms this show has covered. Correct free chlorine, relative to CYA, handles ordinary algae almost every time on its own, which is why you won't find a copper-based algae side on any of our field vehicles for routine algae management. Understanding exactly how copper works is exactly why we know when it's the right tool and when it's the wrong one. And it's also exactly why the one narrow exception on this show, a zero chlorine treatment window during an active metal remediation, calls for a non-metallic polyquat formula instead.

You don't fight a metal contamination problem by adding a new metal to the water. If you're dealing with a metal stain right now, this is the segment you actually came for. So let's walk through the real remediation protocol step by step. I want to explain why each step exists, not just what to do, because understanding the reasoning is what lets you adapt this when your situation doesn't match a case exactly. The first step is confirming which metal you're actually up against, and that has to come from an actual metal-specific test, not a guess based on color alone. Manganese and copper can both drift toward purple depending on concentration and what they've bonded with, so two completely different metals can produce a stain that looks nearly identical to the eye.

Guess wrong here, and every step that follows is built on the wrong foundation. The test kit I actually use in the field is the Jack's Magic Stain Identification Kit. It's built specifically to identify which metal or metals you're dealing with, and you can find it on our supply. Page at poolsscientific.com/slash supply if you need one before you go any further. Once you know what you're fighting, the second step is shutting your chlorine source down completely. Cell, feeder, whatever's driving it, floater. This has to happen before anything else, because chlorine is an oxidizer, which means it's actively working to push that metal into a solid, insoluble form.

If you leave it running while you try to treat the stain, you're fighting your own sanitizer the entire time, redissolving the metal with one hand while chlorine keeps reprecipitating in the other. Nothing else in this protocol works reliably until that fight stops. But shutting chlorine off creates its own problem if you don't account for it. You're about to run several days with little to no active sanitizer in the water, and that's exactly the window algae is waiting for. So ride alongside shutting chlorine down, get a non-copper polyquat-based algacide into the water and keep it maintained through the next few days. This is the one case where we actually recommend it.

A genuine zero chlorine window created on purpose, not ordinary algae management, where correct free chlorine relative to CYA should be doing the job on its own. This dosing of algae side will slow the growth of algae in the water sufficiently until chlorine can be reintroduced later. Skip this step though, and you can come out on the other side of a successful metal treatment looking at a green pool instead, which just adds an entirely separate recovery process on top of the one you were already running. Third, with chlorine out of the picture, bring your pH down to somewhere around 7.0 to 7.2. This isn't an arbitrary number, it's the specific range the next step actually needs to function.

Ascorbic acid, plain vitamin C, is what does the real chemical work here, reducing that oxenized metal back into a dissolved form that you can manage, and it performs that reduction reaction most effectively in this slightly acidic pH range. Skip this step, and your ascorbic acid treatment underperforms no matter how much of it you use. Fourth, treat with ascorbic acid, like we just said, and size the dose to the actual scale of the problem you're looking at at your pool's own volume, not a flat number off of a bag's instructions. This is the step that visibly clears the stain, and it's also the step people mistake for the finish line, which brings me to the part that actually determines whether this holds.

Fifth, and this is the step most people skip or get timing wrong on. Within about 24 hours of that ascorbic acid treatment, get a real metal sequestrant into the water. The one we use in the field is Arenda SC1000, and here's exactly why that window matters. The moment your sanitizer comes back online, that metal wants to oxidize right back out of solution. The exact same reaction you just spent time reversing. Without a sequestrant already in place to hold it, you'll watch the stain reappear within days, sometimes hours, and it'll look like the treatment failed when it really just never finished. Sixth, hold the pool, no swimming, for several days while that sequestrant does its work holding the metal in suspension.

Depending on how much metal contaminant is in your water, this might be as short as two days or as long as a week. Seventh, bring chlorine back slowly once that hold period is over, roughly one part per million per day, rather than dumping it back all at once. Rushing this step is one of the most common ways people undo their own work, since a sudden spike in chlorine can overwhelm the sequestrant's hold before it had time to fully stabilize the metal, sending you right back to where you started again. That's it. The full protocol for metal stain remediation. For most, budget 7 to 10 days for the full treatment start to finish.

Now, if you're dealing with a case involving actual crystal formation, like the copper cyaneuric case we walked through earlier, this same protocol still applies underneath everything else, but it stops being the finish line. You're not just redissolving a stain anymore, you're breaking down a solid compound that's already formed and sitting on your pool floor and inside your filter. That's meaningfully heavier work. That's exactly the point where calling a professional makes more sense than working through these seven steps alone. Every pool, and I mean every pool, eventually deals with some version of what we just covered. Iron is the easiest one, oxidizing reddish brown and usually clearing with ascorbic acid alone, sometimes as simply as cutting a lemon in half and rubbing it directly on the stain, watching it disappear right in front of you.

Manganese is trickier, needing a sequesterant alongside that same ascorbic acid, and it's the one behind more purple and black staining than people realize. Copper is the most versatile troublemaker on the list, blue, green, or purple depending entirely on what it bonds with, and capable of forming an actual crystalline solid when it meets the wrong partner at the wrong concentration. Calcium is a metal too, whether anyone thinks of it that way or not, crossing the exact same kind of threshold as the other three every time your LSI runs positive. Silver and zinc are the two that work for you on purpose when you use them right. One disabling microbes through a mechanism chlorine never touches, the other quietly sacrificing itself so nothing more expensive has to.

That's the whole ecosystem, what each one is, what it actually does in your water, and how you bring every single one of them back under control when they get away from you. You're not guessing anymore. Now you know. So let's say you ran your own numbers tonight and your copper came back at three tenths of a part per million. On its own, that number doesn't tell you much. Is it drifting off a heater that's slowly giving up metal? Sitting well inside of what your cyanuric acid can hold, or already close to the line where the two of them start finding each other. That's the difference between a reading and a read.

And it's exactly what a chemist circle submission gives you. Not just the number back, but what it means for your specific pool, your specific CYA, your specific LSI. What to actually do about it before you're looking at 15 drain and refill cycles like Catherine. Circle members send in real readings twice a month and get a real step-by-step response back, the same kind of thinking you heard in this episode, applied to your water. You can find all the info about it at PoolsScientific.com.

That is the full metals ecosystem. What's in your water, why it moves, and what you do about it when it doesn't stay where it belongs. If tonight helped you make sense of a stain you've been staring at, or a heater that has been eating itself for reasons nobody explained, send this episode to the person who needs to hear it. That's still the best thing you can do for this show and for your pool.

I will see you next week.com. We'll see you next Tuesday.