Oxalic acid works best for pool concrete rust stains.

Yes, but it has to be the right acid and you have to be careful not to mess up your pool chemistry.

Other acids like muriatic or phosphoric can work, but they’re more aggressive on concrete and can mess with your water balance. Oxalic acid (sold as pool rust stain remover or even wood bleach) is the standard - it lifts the rust without eating the surface.

Wear gloves, eye protection, and old shoes. Dilute the powder per the label (usually 1 part acid to 10 parts water). Brush it onto the stain, let it sit 5–10 minutes, scrub, rinse thoroughly. Don’t let it drip into the pool water too fast - test a small area first and make sure your pump is running to dilute the runoff.

If the stain is deep (years of metal furniture sitting in the same spot), you might need a second application. And if it’s really stubborn, a poultice of mixed oxalic acid powder and water left overnight.

Final thought: This is where you buy the acid specifically for pools, not the hardware store version - you don’t want unknown additives in your water.