Water chemistry

Metal Stains in Pools and How to Treat Them

Identify iron, copper and manganese pool stains by color and source, run a quick test and follow a treatment order with a worked example.

Metal stains look like a surface problem. They start in the water. If you treat the stain and ignore the source, they come back. The order is: identify, find the source, balance the water, treat, then prevent.

Identify by color and source

This table points you to a starting guess. A water test for metals confirms it.

Quick stain test

1. Brush the stain first. Algae and dirt often brush off.
2. Organic test: place a small amount of chlorine on the spot, following the stain kit's directions. A stain that lightens is probably organic.
3. Metal test: press ascorbic acid, which is vitamin C, on a spot. A stain that lightens is probably metal.
4. Avoid long contact on vinyl or fiberglass. Strong chlorine can bleach surfaces.
5. Test the pool water and the fill water for metals with a kit or a lab.

Treatment order

1. Find the source. Stains near returns or the heater point to copper. Stains after topping off point to fill water. Stains in shaded corners after leaf drop point to organics.
2. Balance the water. Aim for pH 7.2 to 7.8, TA in range and CH for the surface. Low pH is the main reason copper leaves a heat exchanger. Run the saturation index if the water has been aggressive.
3. Treat the water. Use a sequestrant or chelating product for dissolved metals and a stain remover for existing stains. Dose by the label, scaled to the pool's volume.
4. Expect FC to drop. Ascorbic acid consumes chlorine. Retest FC after treatment and restore it so algae cannot start.
5. Clean the filter. Metals and treatment products can collect there.
6. Retest. Check stains and metals after a week.

Worked example: copper stains on a 12,000 gallon vinyl pool

A vinyl liner pool runs a trichlor feeder. Readings: pH 6.8, TA 50, FC 3. You see blue-green stains near the returns and a green tint in the water. The heater has a copper exchanger. The multiplier is 12,000 / 10,000 = 1.2.

1. Raise TA from 50 to 80, a gain of 30 ppm. 1.4 lb x 3 x 1.2 = about 5 lb of sodium bicarbonate. Split it in two doses.
2. Retest pH after TA moves. If it is still below 7.2, soda ash at about 6 oz per 10,000 gallons raises pH about 0.2. For this pool that is 6 oz x 1.2 = about 7 oz per 0.2. It also raises TA, so go in small steps.
3. Keep pH at 7.4 to 7.6 from here on.
4. Treat with a sequestrant and a stain remover, per the label.
5. Restore FC and retest it.
6. Look at the sanitizer. Trichlor is acidic. Liquid chlorine does not pull pH and TA down the same way. See chlorine types compared.
7. Ask the owner when the heater was last serviced. A copper exchanger that has been eaten by low pH may need a repair, and that is a job for a licensed heater tech.

The raise alkalinity guide has the dose chart for the TA step.

Prevent repeat stains

• Keep pH in range, especially on pools with a copper heat exchanger.
• Test fill water for metals once, and again if the source changes.
• Add a sequestrant when topping off on well water, per the label.
• Test for metals before a heavy shock on pools with a stain history.
• Avoid steel or iron objects in the water. Rusty tools and furniture can leave marks.
• Keep notes of past stains and treatments with each account.

Document before and after

Take photos of the stains and write down the readings before you treat. A before and after record helps with customer questions about cause and cost. If a stain turns into a dispute over damage or who pays, talk to your insurer or a licensed professional. Rules and coverage vary by state. The photo proof guide covers what to capture.

Mistakes

• Scrubbing a metal stain with a hard brush and scratching the surface.
• Shocking a pool with unknown metals in the water.
• Treating the stain and skipping the water balance.
• Using acid directly on a vinyl stain.
• Forgetting to restore FC after ascorbic acid.
• Assuming every dark spot is algae.

Safety

Follow the product label and SDS. Never mix chemicals, including a stain remover and chlorine in the same container. Add chemical to water, never water to chemical. Wear gloves and eye protection.