What causes metal staining in Ottawa pools and how do I remove brown or green stains? | Pool IQ

Metal staining in Ottawa pools is caused by dissolved iron, copper, or manganese in the water that oxidizes and deposits onto pool surfaces when chlorine or pH changes trigger a chemical reaction — and removal methods range from $20 spot treatments to $150 full-pool chemical treatments depending on the severity, type, and age of the stains. Understanding which metal is causing your specific stain colour is the essential first step, because each metal requires a different removal approach.

Iron staining produces brown, rust, reddish-brown, or dark orange marks on pool surfaces. This is the most common type of metal stain in Ottawa pools and can appear as localized spots (often near return jets where chlorinated water contacts the surface), a general brown tint across the entire floor and walls, or distinct rust-coloured patches where metal objects like hairpins, screws, or toys have sat on the liner. Ottawa's source water — whether municipal or well — contains varying levels of dissolved iron. The Ottawa River, which supplies the city's drinking water treatment plants, carries iron from the Canadian Shield watershed, and while treatment removes most of it, trace amounts remain. Rural properties around Ottawa using well water frequently have iron levels of 0.5 to 5.0 ppm — dramatically above the 0.3 ppm threshold where staining becomes likely.

Copper staining produces blue-green, teal, or turquoise marks that are often mistaken for algae. The key difference: algae feels slimy and brushes off easily, while copper stains feel smooth and do not brush away. Copper enters Ottawa pools from several sources. Corroded copper heat exchanger coils in gas or electric pool heaters are the primary source — acidic water (pH below 7.2) dissolves copper from the heater, which then deposits on pool surfaces when pH rises or chlorine oxidizes it. Copper-based algaecides, still sold at Ottawa pool stores despite the staining risk, directly add copper to your water. Well water in certain areas south and east of Ottawa contains natural copper from geological deposits. Even municipal water can carry trace copper from aging copper supply pipes in older Ottawa homes.

Manganese staining produces dark brown, purple, or nearly black marks and is the most stubborn to remove. Manganese is less common than iron or copper in Ottawa's municipal water supply but appears frequently in well water from rural areas including Manotick, Richmond, Vars, and Navan. Manganese stains typically form when the oxidation-reduction potential of the water shifts — which happens every time you add chlorine, shock the pool, or allow pH to change significantly. Even very low manganese concentrations of 0.02 ppm can cause staining over time, making it particularly insidious.

The diagnostic test that identifies your stain type takes 30 seconds and costs under a dollar. Place a vitamin C tablet (ascorbic acid) directly on a stained area and hold it there for 30 seconds. If the stain lightens or disappears under the tablet, it is a metal stain — not algae, not organic, not mineral scale. If the stain does not respond to vitamin C, it may be organic (tannin from leaves or bark), calcium scale (white or grey deposits from high calcium hardness), or a surface defect in the liner or plaster. This vitamin C test works because ascorbic acid is a reducing agent that converts oxidized metals (the coloured, deposited form) back into their dissolved, colourless form. It does not remove the metal from the water — it simply redissolves it temporarily.

Removing iron stains from an Ottawa pool uses the ascorbic acid method at larger scale. For a full-pool treatment, lower chlorine to zero (or as close as possible) by stopping all chlorine input and allowing sunlight and normal consumption to deplete it over 1 to 3 days. Then add 500 grams to 1 kilogram of food-grade ascorbic acid powder per 40,000 litres of pool water, distributed evenly around the pool perimeter. The cost of ascorbic acid powder in bulk from a food-supply or chemical-supply retailer is $15 to $30 per kilogram — dramatically cheaper than buying vitamin C tablets from a pharmacy. Within 24 hours, the ascorbic acid reduces all iron deposits back into solution, clearing the stains. Immediately after the stains clear, add a metal sequestrant at a double dose — $30 to $50 per treatment — to bind the now-dissolved metals and prevent them from re-depositing. Then slowly reintroduce chlorine over the next 48 hours, adding small amounts incrementally rather than shocking, to avoid re-oxidizing the metals faster than the sequestrant can bind them.

Removing copper stains requires a slightly different approach. While ascorbic acid works on copper stains too, a more effective treatment for severe copper staining is a copper stain remover containing citric acid or thioglycollic acid. Products such as Jack's Magic Copper and Scale Stuff or Natural Chemistry StainFree Extra cost $30 to $60 per bottle and are formulated specifically for copper deposits. The treatment process is similar: lower chlorine, apply the stain remover, circulate for 24 hours, then add sequestrant before gradually reintroducing chlorine. For copper stains caused by a corroding heater, the stains will return within weeks unless you fix the root cause — repairing or replacing the heater heat exchanger at a cost of $400 to $1,200 or maintaining pH strictly above 7.2 to prevent ongoing copper corrosion.

Preventing metal stains from returning is more important than the initial removal. Once you have cleared the stains, implement a monthly sequestrant maintenance dose of $20 to $35 per treatment throughout the swimming season. This keeps dissolved metals bound in a form that cannot precipitate onto surfaces. Test your water for metals at least twice per season — at opening and mid-summer — using a comprehensive test from a pool retailer (free at most Ottawa stores). If your source water is the problem (high-iron well water or copper plumbing), consider a hose-end pre-filter for fill water at $30 to $60 per filter that removes metals before they enter the pool.

For Ottawa pools with persistent metal staining problems, a CuLator metal eliminator bag offers a passive, ongoing solution. This product, which costs $25 to $40 per bag, sits in your skimmer basket and continuously absorbs dissolved metals — iron, copper, manganese, lead, and silver — from the water as it circulates through. Each bag lasts approximately 30 days or until it changes colour (indicating it has reached capacity), and it physically removes metals from the water rather than just sequestering them. For an Ottawa pool with known metal issues, using one CuLator bag per month at a seasonal cost of $100 to $160 provides consistent metal removal that prevents stains from forming in the first place.

Professional stain removal services in Ottawa are available if the DIY approach feels daunting or the staining is severe. Most pool service companies charge $150 to $350 for a full stain treatment visit, which includes water testing, chemical application, sequestrant dosing, and a follow-up visit to verify the stains have cleared and chlorine has been safely reintroduced. For plaster-bottom pools (less common in Ottawa than vinyl but found in some older Rockcliffe and Sandy Hill properties), severe staining may require an acid wash — a process where the pool is drained and the surface is scrubbed with a dilute muriatic acid solution to remove embedded stains and a thin layer of discoloured plaster. Acid washing costs $500 to $1,500 and should only be performed by a professional, as it removes plaster material and can only be done a limited number of times before resurfacing is needed.

Dealing with brown, green, or dark stains in your Ottawa pool? Ottawa Pool Installation connects homeowners with local pool chemistry specialists who can identify the metal source, remove existing stains, and implement a prevention plan that keeps your pool surfaces spotless all season.