How to remove copper stains from a pool

Copper dissolved in the water, which almost always comes from inside the pool system rather than from the hose. The usual sources:

• The heater. Many heat exchangers are copper. Water with the pH running low slowly eats at them, and the copper ends up on the plaster.
• Copper algaecides. Used once in a while they are fine. Used every week, the copper builds up.
• Ionizers and mineral systems that add copper on purpose.
• Low pH in general. Acidic water pulls metal out of anything it touches.

Well water in Palm Beach County brings iron far more often than copper. If your stain is rust brown rather than blue green, see how to remove well water stains.

Blue green, teal, turquoise or gray patches on the plaster, often near the return jets, on the steps or across the floor. In pools with a lot of stabilizer, copper can also show up purple or violet. Copper in the water can tint blonde hair green before it ever stains a wall.

The water itself is a clue too. A pool that turns clear green right after a shock, rather than cloudy green, usually has metal in it. More on that in why is my pool green.

Do the vitamin C test. Crush a vitamin C tablet, put it in a sock or hold it right against the stain for about 30 seconds, then look. If the spot underneath went lighter, the stain is metal. Algae and leaf stains do not react to vitamin C, but they do fade with chlorine and brushing.

The stain color guide has the other colors and what each one usually means.

Lift them with ascorbic acid, which is vitamin C in bulk, with the pool full. The short version: lower the chlorine, lower the pH, add ascorbic acid with the pump running, brush, then lock the copper up with a sequestrant before the chlorine goes back up. The numbered steps are below.

Read the label on the product you buy, because the doses differ between brands. And add one chemical at a time. The CDC is clear that pool chemicals should never be mixed with each other.

Yes. Most copper stains come off with ascorbic acid while the pool is full, and that is where to start. Draining only makes sense when the stain has set deep into old plaster, or the plaster is worn out anyway. Draining a Florida pool has its own risks, covered in should I drain my pool.

Usually, because an acid wash takes off a thin layer of plaster and the stain goes with it. But it is the last resort, not the first. Try ascorbic acid first, and find the copper source before any wash, or the copper still in the water just stains the fresh surface.

If you get to that point, our acid wash is a drain, wash and refill, and acid wash or resurface explains whether your plaster has a wash left in it.

Keep the pH from running low, stop adding copper, and keep a sequestrant in the water.

• Hold the pH in range. The CDC range is 7.0 to 7.8. A pool that sits below that is dissolving its own heater.
• Switch away from copper algaecides if you use them every week.
• Add a metal sequestrant after the treatment and top it up on the schedule on the bottle.
• Test regularly. Low pH does its damage quietly. That is part of why a weekly pool service tests and balances the water every visit.