Most brown and blue-green stains on pool plaster are metal, and a vitamin C tablet held against one for a minute confirms it. If a clean spot appears, the color then names the metal: brown for iron, blue-green for copper, purple-black or gray for manganese.
Identification matters because the treatments are opposites. Get it right and most metal stains lift off in a weekend; shock a pool with metal in the water and you can set the stain permanently into the surface.
Metal stains often get diagnosed as dead plaster. A homeowner sees rust-colored discoloration spreading across a white wall, assumes the finish is finished, and gets a five figure resurfacing quote for a problem that a bottle of ascorbic acid would have solved. The color was the clue. The EPA’s guidance on secondary drinking water standards says corroding iron and copper “may stain household fixtures and impart objectionable metallic taste and red or blue-green color to the water supply,” and plaster takes the same colors. Before anybody talks about resurfacing, establish what you’re looking at.
Metal stain or organic stain
Pool stains come in two families that behave in opposite ways. Organic stains come from leaves, algae, acorns, worms, and anything else that decayed against the surface. Metal stains come from dissolved metal in the water that oxidized and plated onto the plaster.
| Organic | Metal | |
|---|---|---|
| Common shape | Leaf-shaped, blotchy, follows where debris sat | Streaks below fittings, rings, broad even patches |
| Common color | Brown, green, reddish brown | Rust brown, teal, purple, gray |
| Responds to chlorine | Yes, lightens or lifts | No, usually darkens |
| Responds to ascorbic acid | Barely | Lifts, often visibly within a minute |
| Where it appears first | Floor, corners, under the steps | Below returns, around the light, at the waterline |
The vitamin C test
For the vitamin C test, hold a plain vitamin C tablet (ascorbic acid) against the stain underwater for thirty to sixty seconds. If a clean spot appears in the shape of the tablet, it’s a metal stain and ascorbic acid is going to be part of the answer. If nothing happens, try the chlorine version: hold a trichlor tablet against the stain the same way, with gloves on. If that lightens it, the stain is organic and your problem is a shock and brush job rather than a metal one. Test on the shallow end wall, where a small clean patch won’t show, rather than on the most visible stain in the pool.
Reading the color
Each color points to a different metal, and each metal to a different source.
| Color | Metal | Usual source | Notes |
|---|---|---|---|
| Rust brown, orange, tan | Iron | Well water fill, corroding steel, iron-rich fill | The most common here. Often appears after a big refill. |
| Blue-green, teal, aqua | Copper | Heater heat exchanger, copper algaecide, corroding plumbing | Usually generated by your own pool rather than delivered to it. |
| Purple, black, dark brown-gray | Manganese | Well water, some municipal sources | Less common, stubborn, and often mistaken for a dark organic stain. |
| Blue-black waterline smudge | Copper plus scale | Copper in hard water at high pH | Two problems layered, and the scale has to come off first. |
Do not shock a pool with metal in the water
Chlorine is an oxidizer. Dissolved metal sitting invisibly in your water is oxidized by a shock dose and comes out of solution onto the nearest surface, which is your plaster. Anybody who has refilled from a well, shocked the pool the same day, and found the whole shallow end brown the next morning has run this experiment. Sequester first, shock later.
Where the metal came from
Iron, and why fill water matters
Most iron arrives dissolved in the water you added. Plenty of properties on the outskirts of Collin County, out toward Celina, Prosper, Lucas, and Parker, run on wells or have a well for irrigation that gets used for the pool because it isn’t metered. Well water in this region can carry meaningful iron and manganese, and it goes into the pool clear, invisible until it oxidizes.
Iron staining often shows up right after a lot of new water: a partial drain and refill, a leak that ran for weeks with the autofill compensating, or a first fill on a new pool. If you top off from a well, you’re adding iron every time.
There is a useful number for how little it takes. The EPA’s secondary drinking water standards are non-mandatory guidelines for taste, color and staining rather than health limits. Iron is listed at 0.3 mg/L and manganese at 0.05 mg/L, the levels above which the EPA lists reddish or orange staining for iron and black staining for manganese.
Federal rules don’t require a water system to treat for them, and a private well has no obligation to meet them. A third of a milligram per liter is a level you cannot see, taste, or notice in a glass of water. Now put twenty thousand gallons of it in a pool and oxidize all of it onto one wall.
Copper, and the salt pool chain
Copper is different: it’s usually manufactured inside your own equipment. The three sources, in order of how often we find them:
- A gas heater’s heat exchanger. The exchanger is usually copper. Aggressive water passing through it dissolves it, and the pool gets both a stain and, in time, a failed heater. Water that is corroding the exchanger is the same water described in why your heater is not heating.
- Copper-based algaecide. It works, and it deposits copper in your pool permanently. Used once on a green pool recovery it’s often fine. Used routinely as a maintenance product it accumulates and eventually plates out.
- A mineral or ionizer system. These deliberately put copper in the water. Staining is a known tradeoff, not a malfunction.
On a salt pool, the salt cell generates chlorine continuously and pushes pH up, so the owner adds acid to hold it down. Someone overshoots, and the water sits acidic and low in alkalinity for weeks. The heater’s copper exchanger dissolves into the water, and the first sign anybody notices is a teal stain spreading under the returns. It’s the ordinary result of running a salt pool without watching alkalinity, and it’s covered from the other end in salt water pool maintenance.
Removing the stain
The standard ascorbic acid treatment below is effective on iron, usually effective on copper, and hit or miss on manganese. Read all of it before starting, because the order is what makes it work.
- Test and record everything first. Free chlorine, pH, alkalinity, calcium, cyanuric acid (CYA, sold as stabilizer), and a metals test if you can get one. Write it down. You’re going to be putting the pool back afterward.
- Get chlorine to near zero. Ascorbic acid and chlorine consume each other, so a pool at 5 ppm will eat the treatment. Stop chlorinating and let the sun bring it down over a day or two, or use a neutralizer.
- Bring pH down toward 7.2. Metal lifts more readily from a slightly acidic surface.
- Turn off any salt cell and the heater. A cell running through this treatment is making chlorine you’re trying to remove, and the heater has copper you are actively trying not to dissolve more of.
- Add ascorbic acid with the pump running. Broadcast it across the surface. On iron staining the change is often visible within thirty minutes.
- Circulate and brush. Give it several hours, brushing the stained areas. Repeat the dose if some remains.
- Add sequestrant immediately. This is the step people skip and it is the one that decides whether the stain stays gone. The metal is now dissolved in your water rather than on your wall, and it will plate straight back out the moment chlorine returns unless it’s bound up first.
- Bring the pool back slowly. Restore pH and alkalinity, then bring chlorine up over several days rather than in one dose. A shock at this point undoes the whole job.
- Keep dosing sequestrant on a schedule. It degrades. Most products want a monthly maintenance dose, and if you fill from a well, treat every refill.
What sequestrant does and does not do
A sequestrant chemically binds dissolved metal so chlorine cannot oxidize it out of solution. It does not remove the metal, which stays in the water, held in suspension; that is why the dosing continues. A pool with a lot of accumulated metal eventually needs the water partly replaced or treated by reverse osmosis rather than managed forever with a bottle.
Stopping it from coming back
- Filter the fill. A hose-end filter for pool fills is inexpensive and catches iron before it enters. If you fill from a well, this is the highest-value item on the list.
- Hold alkalinity where it belongs. Copper staining usually follows water that spent time aggressive. The targets and the reasoning are in the water chemistry guide.
- Stop using copper algaecide as routine maintenance. If algae is a recurring problem, the cause is sanitizer and circulation, and it’s worth reading the algae types guide rather than adding metal to the water every month.
- Fix the reason for the big refills. A pool that needs a lot of new water is often a pool with a leak, and every new gallon brings more iron. Work through leak or evaporation if you’re unsure which you have.
- Watch the heater. A heat exchanger that has started shedding copper is telling you about the water it’s sitting in, and it’s a repair on a clock.
When it will not come off
Some stains have been there long enough, or through enough shock cycles, that they’ve become part of the surface. Manganese in particular resists ascorbic acid more often than iron does. At that point there are two outcomes.
The first is living with it. A faded metal stain on a pool that is otherwise sound is a cosmetic issue, and it will fade further under ordinary use. The second is an acid wash, which strips a layer of plaster and takes the stain with it. It works by spending part of the remaining life of your finish, a trade to make deliberately. If the surface is already thin, you are looking at a resurface sooner rather than a stain treatment now.
Austin’s tip
Photograph the stain from a fixed spot on the deck, at the same time of day, before you start and every day through the treatment. Metal stains lift too gradually to judge by eye. Two photos a day apart settle it, and they also tell you whether it is creeping back three weeks later while there is still time to dose sequestrant rather than repeat the whole treatment.
What we would do with a stained pool
Lifting the stain is half the job. The other half is finding the source, because iron arriving in a well fill and copper leaving a dissolving heat exchanger will both keep producing the same stain until somebody interrupts them.
The work we would rather be doing is duller than any of that. A metals test on a pool that gets topped up from a well, and an alkalinity reading held where it belongs on a pool with a gas heater, are five-minute jobs that prevent this whole problem. That is what weekly service is for, and it costs a great deal less than a weekend of ascorbic acid and a pool you have to rebuild the chemistry on afterwards.
