Pool stains: identify and remove

Identify: vitamin C and chlorine spot tests

Treating the wrong kind of stain wastes money and can make it worse: shocking a metal stain can darken it, and acid on an organic stain does little. Start by looking, then test. PHTA suggests combining the stain's history, your water test, and a spot test with ascorbic or citric acid.

1. Test free chlorine, pH, and (if you have a kit) copper and iron. Note whether you recently filled from a well, used a copper algaecide, or had very low pH.
2. Vitamin C test: put a vitamin C (ascorbic acid) tablet in a thin sock or hold it with a gloved hand, press it on the stain for about 30 seconds, then check. If the spot lightens, it's metal.
3. Chlorine test: hold a trichlor tablet in a sock against the same stain for 30–60 seconds (only briefly on vinyl; it bleaches liners). If the spot lightens, it's organic.
4. If neither works: rub it with your fingers. Rough, raised, and whitish means scale.

One catch from PHTA's metals bulletin: stains caused by iron bacteria can "disappear" with ascorbic acid but return as soon as chlorine is restored. If a "metal" stain keeps coming back after treatment, suspect organic causes too.

Metal stains (iron, copper)

Metals get into pools from more places than most owners expect. PHTA's committee lists fill water (especially untreated well water), iron-bearing stone such as flagstone and granite that leaches after rain, copper heat exchangers in gas heaters when water is out of balance, copper algaecides and mineral systems, fertilizer runoff, and galvanic corrosion when the pool's bonding system doesn't work.

Why is the water or surface brown? Usually iron. Water from a well can look clear coming out of the hose, then turn brown, green-brown, or tea-colored once chlorine oxidizes the dissolved iron. Well water can also carry iron bacteria, which PHTA links to brown water and staining. Tannins from leaves give a similar tea color without any metal.

Green hair is copper, not chlorine. Copper ions bind to hair, and hair that is already damaged by chlorine, bleaching, or sun takes it up most, according to researchers writing in The Conversation.

How to remove metal stains, following PHTA's three steps:

1. Lift the stain with an ascorbic acid, citric acid, or similar stain remover added to the water or applied to the spot, following the label. Ascorbic acid reacts with chlorine, so these treatments usually mean lowering free chlorine for a few days with no swimming; the label gives the exact procedure.
2. Bind the dissolved metal with a sequestering agent (metal chelator) so it doesn't oxidize and redeposit. Sequestrants are broken down by chlorine, so they need regular maintenance doses.
3. Get the metal out: replace part of the water with metal-free water, use reverse osmosis, or use insoluble adsorbent products (for example filter pads or media that bind metals). PHTA notes sequestrants alone don't remove metals, despite "metal remover" labels.

Leave muriatic acid treatment of surfaces to experienced professionals; PHTA warns it can permanently damage the finish.

Organic stains

Leaves, acorns, berries, worms, and algae leave green, brown, or pinkish marks, usually right where the debris sat. They respond to chlorine: brush the spot while free chlorine is at the top of your range, or hold a chlorine tablet in a sock against it briefly (not on vinyl for long). Keep debris off the floor, run the pump long enough to keep it moving, and hold chlorine at the level your stabilizer needs. A green tint across the whole pool is algae, not a stain; see green pool.

A strong "chlorine" smell after a storm brings in debris doesn't mean too much chlorine; it's usually combined chlorine from organic waste (pool smells like chlorine).

Scale (LSI too high)

Calcium scale is white to gray, rough, and builds at the waterline, on tile and around returns where water is warmest and splashes most. It forms when the water is oversaturated with calcium carbonate: high pH, high alkalinity, high calcium, warm water. The LSI calculator (Langelier Saturation Index) shows how far over you are; above +0.3, scale is likely, and hot spas scale first.

• Lower pH toward 7.2–7.4 and total alkalinity (TA) toward 60–80 ppm to stop new scale.
• Remove light waterline scale with a tile-and-scale cleaner, or a pumice stone on ceramic tile only.
• Heavy scale on plaster: call a professional.
• If calcium itself is high, see calcium hardness and plan dilution.

Prevention

• Test your fill water. PHTA recommends treating fill water with total metals above 0.2 ppm before it goes in, and avoiding untreated water above 1 ppm. A hose-end pre-filter for metals helps when topping up from a well.
• Keep pH at 7.2 or above. PHTA calls pH the most significant and most overlooked factor in keeping metal from dissolving out of heaters and equipment; water under pH 7 is likely corrosive.
• Skip copper algaecides if you can hold chlorine at the right level instead. CDC guidance cited by PHTA keeps copper at or below 1.3 ppm, and staining can start lower.
• Keep stabilizer (CYA) moderate. Copper and high CYA can form a purple copper cyanurate stain (PHTA).
• Have the bonding system checked by a licensed electrician if you see corrosion on ladders, lights, or rails; PHTA recommends testing it at least yearly.
• Test metals with a two-step total-metal kit if you can; PHTA notes one-step tests often miss sequestered or oxidized metal.

When to call a pro: stains that don't respond to either spot test, stains that return right after treatment, large areas of discoloration on plaster, or any sign of electrical or bonding problems. More fixes: pool water problems.

FAQ

Why is my pool brown?

Most often dissolved iron that turned brown when chlorine oxidized it, typically after filling or topping up from a well. Tannins from leaves and iron bacteria cause brown water too. Test for iron, hold off on shocking until the metal is sequestered, and follow the metal-removal steps above. Brush and filter out any oxidized particles already floating.

What causes pool stains?

Metals (iron, copper, manganese) from fill water, equipment corrosion, or copper algaecides; organic debris such as leaves, berries, and algae; and calcium scale from water with a high saturation index. A vitamin C and a chlorine spot test tell you which one you're dealing with.

What causes yellow pool stains?

Yellow patches that brush off and come back in shady spots are usually mustard algae. Fine yellow dust on the surface and steps in spring is often pollen. Yellow-brown spots that lighten with vitamin C point to iron. Check pool algae types if it brushes off easily.

Sources

• PHTA Recreational Water Quality Committee, Metals and metal staining (AQUA Tech Notes, August 2020): phta.org (PDF)
• PHTA fact sheet, Source water and its effects on pool and spa maintenance (2020): phta.org (PDF)
• PHTA technical bulletin, Water Balance Indexes: phta.org (PDF)
• The Conversation, Why does my hair turn green from the swimming pool? (2023): theconversation.com