Iron and Copper in Your McKinney Pool — What's Causing the Stains
Brown water after shocking? Green stains on the plaster? Metal contamination is more common in McKinney pools than you'd expect. Here's why.
You shocked the pool last night and woke up to brown water. Not green — brown. Or maybe you've noticed a teal-green ring forming on the plaster around your waterline that no amount of scrubbing removes. Before you blame your filter or pour in more chlorine, consider something most McKinney pool owners never test for: dissolved metals.
Iron and copper are invisible when dissolved in pool water. You can't see them, smell them, or taste them. But the moment you add chlorine — a powerful oxidizer — dissolved iron converts to ferric iron (rust-colored particles) and dissolved copper can precipitate into copper carbonate or copper sulfate (green or blue-green deposits). The shock you added to improve the water is the very thing that made the metals visible.
McKinney sits at an interesting intersection for this problem. The city has both municipal water-fed properties and a smattering of older homes on the outskirts with legacy plumbing. The neighborhoods around Craig Ranch, Stonebridge Ranch, and the historic downtown area have pools spanning multiple decades, some with original copper heat exchangers, copper plumbing segments, and older equipment that leaches metals into the water over time.
Where the Metals Come From
Source 1: Your Fill Water
McKinney's municipal water from the North Texas Municipal Water District contains trace metals — it's treated to drinking water standards, but "trace" isn't "zero." Iron at 0.05-0.2 ppm and copper at 0.01-0.1 ppm are typical in treated municipal water. These levels are safe for drinking but become problematic in a pool that concentrates minerals through evaporation.
A pool topped off regularly through summer can accumulate iron to 0.3+ ppm and copper to 0.2+ ppm — levels where oxidation from shock treatments produces visible discoloration.
Source 2: Pool Equipment
This is the bigger source in McKinney's older pools. Copper appears in pool systems in several places:
- Copper heat exchangers in older pool heaters — the most common source. Water flowing through a copper heat exchanger picks up trace copper continuously, especially if pH is low (acidic water accelerates copper dissolution).
- Copper-based algaecides — products containing copper sulfate or chelated copper intentionally add copper to the water. Used once or twice, the copper level stays manageable. Used weekly all summer, it accumulates.
- Older copper plumbing segments — homes built before the mid-2000s may have copper pipe sections between the equipment pad and the pool. These corrode slowly and release copper into the circulation system.
Iron enters primarily through fill water, but older steel or galvanized fittings in the plumbing can also contribute.
Source 3: Environmental Sources
Runoff from McKinney's clay soil can introduce iron. The red-orange clay common throughout Collin County is iron-rich. After heavy rains, particularly in neighborhoods like Stonebridge Ranch where the terrain has more elevation change, iron-laden runoff can enter pools directly or through saturated soil that contacts the pool shell.
Identifying the Metal by Color
The stain or discoloration color tells you which metal is responsible:
Brown, rust, or reddish-brown = Iron. This appears as rusty-looking water after shocking, brown stains on the pool floor (especially in low-circulation areas), or a rust-colored ring at the waterline. Iron stains darken over time if untreated.
Green, teal, or blue-green = Copper. This appears as green staining on lighter plaster surfaces, green discoloration around returns and fittings, or — in severe cases — green-tinted hair on blonde swimmers. Copper stains can look similar to algae but won't brush off easily and have a more metallic sheen.
Black or dark gray = Manganese or high-concentration iron. Less common but occasionally found in McKinney pools, particularly those receiving water from older municipal mains or from supplemental well sources.
The Fix: Sequestration and Source Control
You can't filter dissolved metals out of pool water — they pass right through any pool filter in dissolved form. And you can't shock them away — shocking is what oxidizes them into visible particles in the first place. The approach is a two-step strategy: manage what's already in the water, then reduce what's coming in.
Step 1: Metal Sequestrant
A metal sequestrant (also called a chelating agent) binds to dissolved metals and keeps them in solution — preventing them from oxidizing into visible stains or particles. Key products include CuLator (which physically absorbs metals), Jack's Magic Blue Stuff or Pink Stuff (liquid sequestrants), and Natural Chemistry Metal Free.
How to use: Add the sequestrant at the label-recommended dose with the pump running. Give it 24 hours to circulate and bind with the metals. Don't shock the pool during this period — shocking oxidizes metals and works against the sequestrant.
After 24 hours, resume normal chlorine maintenance. The sequestered metals will either remain bound in solution (harmless) or gradually get captured by the filter as they complex with the sequestrant.
Maintenance dosing: In pools with ongoing metal introduction (from equipment, fill water, or environmental sources), a maintenance dose of sequestrant every 2-4 weeks prevents accumulation from reaching visible levels.
Step 2: Identify and Address the Source
Sequestration manages the symptom. Eliminating the source is the long-term fix.
If copper is coming from your heater: Test for copper immediately after running the heater for several hours. If copper levels rise after heating, the heat exchanger is corroding. Options: replace the heat exchanger ($800-1,500 depending on the heater), upgrade to a heater with a cupro-nickel or titanium exchanger, or use a sequestrant continuously and accept the ongoing copper introduction.
If copper is coming from algaecide: Switch to a non-copper algaecide (polyquat-based products contain no metals) or eliminate algaecide entirely if your chlorine maintenance and shocking routine is adequate to prevent algae.
If iron is coming from fill water: Use a hose-end pre-filter with an iron-removing cartridge when filling or topping off the pool. Products from CuZn, Pre-Fresh, and similar brands remove a significant percentage of dissolved iron before it enters the pool. Cartridges cost $25-40 and last about 2,000-3,000 gallons.
If metals are from old plumbing: Have a plumber inspect the pipe runs between the equipment pad and the pool. Copper segments can be replaced with PVC. This is a one-time fix that eliminates the source permanently.
Removing Existing Metal Stains
If stains have already formed on your plaster, waterline tile, or steps, sequestrant alone won't remove them — it prevents new stains but can't lift existing deposits. Stain removal requires a different approach depending on the metal.
Iron Stains
Ascorbic acid (vitamin C) is remarkably effective at lifting iron stains from plaster and tile. You can test this cheaply: crush a vitamin C tablet and hold it against the stain for 30 seconds. If the stain lightens or disappears, iron is confirmed and ascorbic acid treatment will work.
For a full pool treatment, add 1 pound of ascorbic acid per 10,000 gallons with the pump running. The stains should lift within hours. After treatment, add a metal sequestrant immediately to bind the now-dissolved iron and prevent it from re-depositing.
Important: Ascorbic acid will temporarily crash your free chlorine to near zero. Don't shock for 48 hours after treatment — the chlorine will oxidize the iron right back onto the surfaces. Let the sequestrant work first, then gradually rebuild chlorine.
Copper Stains
Copper stains are harder to remove than iron. Ascorbic acid has limited effectiveness on copper. For light copper staining, a citric acid treatment (similar process to ascorbic acid, 1 pound per 10,000 gallons) can lighten the deposits.
For heavy copper staining on plaster — the deep green discoloration that's common in pools with old copper heat exchangers — professional acid washing may be required. This involves partially draining the pool and applying a muriatic acid solution directly to the stained surfaces. It's effective but removes a thin layer of plaster each time, so it shouldn't be done more than once every few years.
Ongoing Metal Management for McKinney Pools
Given McKinney's mix of pool ages and water sources, a practical metal management protocol looks like this:
- Test for metals annually — at minimum for iron and copper. Bring a water sample to a pool store that offers a comprehensive metal panel, or use a mail-in test kit.
- Use a sequestrant year-round if metals are present at detectable levels. Monthly maintenance doses prevent accumulation.
- Pre-filter fill water if your tap water test shows iron above 0.1 ppm or copper above 0.05 ppm.
- Avoid copper-based algaecides in any pool with existing copper issues.
- Check pH weekly. Low pH (below 7.2) accelerates metal corrosion from equipment and plumbing. Keeping pH at 7.2-7.6 reduces the rate of copper and iron dissolution.
- Inspect equipment during annual service. Look for green patina on copper fittings, rust on steel components, and signs of heat exchanger degradation.
For pools in Craig Ranch and Stonebridge Ranch that are approaching 10-15 years old, the combination of aging equipment and concentrated minerals makes proactive metal management increasingly important. Waiting until stains appear means the metals have been accumulating for months — catching it early with regular testing saves the cost and hassle of stain removal.
Mysterious stains showing up in your McKinney pool? Hydra Pool Services tests for metals, manages water chemistry, and keeps stains from forming — serving McKinney, Frisco, Plano, Allen, Murphy, Parker, and The Colony. Request a water analysis →