How to Remove Pool Stains

How do you tell what kind of pool stain you have?

Match the color, then run a two-minute spot test. Organic stains are usually green, brown, or tan and tend to sit where leaves or debris collected. Metal stains follow the metal: iron reads rust brown, copper reads blue-green or teal, and manganese reads purple to nearly black. Color alone is not proof, though, because iron and leaf tannins can both look brown.

Two cheap tests settle it. Hold a trichlor chlorine tablet against the stain for about 30 seconds; if the spot lightens, it is organic. For a suspected metal stain, press a crushed vitamin C (ascorbic acid) tablet onto it; if that spot fades, it is metal. Test an inconspicuous area first, and remember you can have both kinds in the same pool.

The reason to test is that chlorine and ascorbic acid work against each other. Chlorine oxidizes and removes organic stains, but it also oxidizes dissolved metal and drives it onto surfaces, which is why a metal-stained pool looks worse right after a shock. Ascorbic acid does the reverse. Getting the diagnosis right saves you a wasted, expensive treatment.

How do you remove organic pool stains?

Brush the stain, then raise chlorine and hold it. Organic material is carbon based, so oxidizing it with chlorine breaks it down and the stain lifts, usually within a day or two.

Balance the water first, then bring free chlorine up with a shock dose and brush the stained areas hard. Run the pump to keep chlorine circulating, and rebrush daily until the stain is gone. Use the pool shock calculator to size the dose for your gallons. A stubborn oily stain in a corner can also respond to a pool enzyme product, which digests the residue.

If the stain is a yellow or dull film on floors and steps that keeps coming back, treat it as mustard algae rather than a plain stain. And if chlorine does nothing after a solid shock and good brushing, stop; it is almost certainly a metal stain, and more chlorine will only make it harder to remove.

How do you remove metal stains with ascorbic acid?

Use ascorbic acid, better known as vitamin C, and get the chlorine out of the way first. Ascorbic acid converts the stain back into dissolved metal so it releases from the surface.

Start by lowering free chlorine to near zero, because chlorine re-oxidizes the metal and cancels the treatment. Lowering chlorine with sun and time, or a neutralizer, is the safe route. Turn off any salt generator or tablet feeder, then broadcast ascorbic acid over the pool, or hold it against a localized stain in a sock. Follow the product label for the amount and add more as needed rather than dumping it all at once. Stains often fade within minutes to a few hours.

Removing the stain only puts the metal back into the water, so it will re-stain unless you lock it up. Add a metal sequestrant (chelating agent) right after the acid treatment to hold the metal in solution, run the filter, and bring chlorine and pH back to normal slowly over the next day or two. Skipping the sequestrant is the most common reason a metal stain comes straight back.

Will muriatic acid remove pool stains?

Sometimes, but it is rarely the right tool. Muriatic acid dissolves calcium scale and can lighten some mineral deposits, so a diluted spot treatment or a full drain-and-acid-wash is used on plaster pools with heavy scaling.

The catch is that an acid wash strips a thin layer off the plaster every time, so it is a last resort you can only repeat so often before the surface needs resurfacing. It also does little for true metal or organic stains, which ascorbic acid and chlorine handle without draining or etching. On a vinyl or fiberglass pool, skip muriatic acid on stains entirely, because it can bleach and damage the surface.

Can you remove pool stains without draining?

Yes. Both main treatments, chlorine for organic stains and ascorbic acid for metal stains, work in a full pool with no draining at all. Draining is not part of normal stain removal.

You only drain for the heavy cases: a plaster pool that needs an acid wash, or water so loaded with metal that treating it in place would just re-stain over and over. Even then, a partial drain and refill to dilute the metal is often enough, the same approach used to lower cyanuric acid. Draining a pool carries real risks, from liner shrinkage to a fiberglass or concrete shell floating out of wet ground, so treat a full drain as the exception.

How do you keep pool stains from coming back?

Control metals and keep the water balanced. Most recurring stains trace back to metal in the water or scale from unbalanced chemistry, and both are preventable.

If you fill from a well or from metal-rich municipal water, test the fill water and add a sequestrant on a regular schedule, especially at opening and after topping off. A sequestrant keeps iron and copper dissolved so they never plate onto surfaces. Copper often comes from a copper-based algaecide or from acidic water eating at copper heater parts, so watch your pH and go easy on copper algaecides.

For scale and mineral staining, keep the water in balance. Hold calcium and pH in range and check the saturation index, which tells you whether your water will scale or stay clear; use the calcium hardness calculator to dial in hardness. Finally, get leaves, worms, and berries out promptly, because organic debris left sitting is what starts most organic stains.

Frequently asked questions