Pool stains: how to tell metal from organic, and how to remove them
Most pool stains are either organic (leaves, berries, algae, dirt) or metal (iron, copper, manganese). They need opposite treatments, so the first job is figuring out which you have. Two cheap spot tests usually settle it.
Spot test 1: chlorine
Put a trichlor tablet or a little granular chlorine in an old sock, and hold it against the stain for 30 seconds or so. On vinyl, use a short contact time and keep it moving, since concentrated chlorine can bleach liners.
- Stain lightens or disappears: it's organic. Leaves, pollen, berries, worms and algae leave greenish-brown, brown or reddish-brown stains.
- No change: probably metal, so try the second test.
Spot test 2: vitamin C (ascorbic acid)
Press a vitamin C tablet against the stain, or hold a sock with crushed tablets against it, for 30 seconds.
- Stain lifts: it's a metal stain, most often iron (brown, rust or orange-brown) or copper (blue-green, teal, sometimes gray or black). Ascorbic acid dissolves these metal deposits.
- No change with either test: it may be scale, mineral deposits, a stain set into old plaster, or something in the surface itself. A pool professional can help identify it.
Organic stains
Treat them by raising free chlorine and brushing. Bring chlorine up to the shock level for your stabilizer with the shock calculator, brush the stains, and keep chlorine up until they fade. Stubborn spots on plaster can be spot-treated with chlorine in a sock. Then keep leaves out, run the skimmer, and keep chlorine at the right level for your CYA so the stains don't return.
Where metal stains come from
- Iron: well water and some municipal water. Water may look clear until chlorine is added, then turns tea-colored or greenish and stains.
- Copper: copper-based algaecides, copper ionizers, or corrosion of copper heat exchangers and fittings, especially when pH runs low or the water is corrosive (negative LSI).
- Manganese: some well water. Often purple, black or brown.
- Metal objects: hair pins, nails, coins or tools left on the floor leave small rust spots.
Removing metal stains
Metal stain treatments usually follow this pattern. Follow the instructions of the product you use, since details differ:
- Lower free chlorine to near zero first. Ascorbic acid is used up by chlorine, and the chlorine would re-oxidize the metals.
- Add ascorbic acid (or a commercial stain remover based on it or on other acids), spread over the stained areas, with the pump running.
- Add a metal sequestrant to keep the dissolved metals in solution so they don't redeposit.
- Rebalance slowly. Ascorbic acid lowers pH and alkalinity, so restore those, then bring chlorine back up gradually over a few days. Raising chlorine quickly can bring the stains straight back.
Afterward, keep adding sequestrant as its label directs, especially when topping up with well water, and fix the source: switch away from copper algaecides, keep pH and LSI in range, and consider a pre-filter for metal-heavy fill water.
Preventing stains
- Test fill water for metals, especially well water, and use a sequestrant or hose pre-filter if they're present.
- Keep pH and the saturation index in range, so the water doesn't corrode metal equipment. The pH calculator helps.
- Skim leaves before they sink, and keep chlorine at the right level for your stabilizer.
- Don't add chemicals as undissolved piles on the floor. Some granular products can bleach or stain the spot where they settle.
Frequently asked questions
Why did my pool turn green right after I shocked it?
If clear water turns green, brown or tea-colored right after adding chlorine, metals in the water (often iron or copper) have oxidized. That's different from algae: the water usually stays clear but tinted, rather than turning cloudy. Test for metals and use a sequestrant before adding more chlorine.
Will a metal stain remover damage my pool?
Used as directed, these products are designed for pool surfaces, but they lower pH and alkalinity, and some need chlorine to be very low during treatment. Read the label, and rebalance afterwards.
Can I use a vitamin C tablet from the pharmacy?
For the spot test, yes. For treating a whole pool, pool-grade ascorbic acid products are more practical because of the quantities involved.