Pool Stain Types and How to Identify Them

Stains in pool plaster, vinyl, or fiberglass have specific causes. Treating them blindly with stain remover or shock often makes things worse. The right approach starts with identification — the color and pattern of the stain tells you what caused it and what removes it.

The four main stain types

1. Metal stains (most common)

Iron stains are brown to rust-colored, copper stains are bluish-green to black, and manganese stains are pink to purple. Cause: well water with dissolved metals, or copper from heat exchanger corrosion getting into the pool.

Treatment: Add a metal sequestrant to bind the metal back into solution. Brush the stain. Apply ascorbic acid (vitamin C) directly to small stains. For widespread metal stains, an acid wash by a service tech is sometimes needed.

2. Organic stains

Brown to reddish-brown blotchy stains, usually under leaves, acorns, or other organic matter that sat on the pool floor for weeks. Common at spring opening on pools closed with a leaf-collecting cover.

Treatment: Shock to 10–15 ppm free chlorine. Brush the stain. Most organic stains lift within 24 hours of shocking.

3. Algae stains (residual)

Greenish to dark green spots where algae was killed but left behind chlorophyll staining. Most common on plaster after a green-pool recovery.

Treatment: Triple-shock, brush daily, run pump 24/7. Algae stains typically lift within a week with aggressive chemistry.

4. Calcium / mineral scale

White to gray crusty deposits, usually at the waterline tile, on heater elements, or salt cell plates. Cause: high calcium hardness + high pH.

Treatment: Lower pH and calcium. For existing scale: tile cleaner with citric acid, or mechanical brushing.

The stain identification test

Take a vitamin C tablet and hold it directly against the stain underwater for 30 seconds. If the stain lightens, it’s a metal stain (iron is most likely). If no change, it’s organic, calcium, or algae.

Chemicals for treating common stains

Prevention

• Use a metal sequestrant if you fill with well water — binds dissolved metals before they stain.
• Keep pH at 7.4–7.6 and calcium at 200–400 ppm — prevents scaling.
• Skim leaves immediately — prevents organic stains during fall.
• Brush weekly — prevents algae from anchoring.

If your stain doesn’t match any of these descriptions, send PST Pool Supplies a photo and a water test result. We can usually identify it from the photo alone and recommend the right treatment.