Pool stains: how to identify and remove them

Green, brown, black, blue or white stains in the pool: how to tell the cause of each one and the right way to remove it.

2 min read

Before you scrub, work out what kind of stain it is. Each cause needs a different treatment, and the wrong product can make it worse.

Quick test: organic or metal?

Rub a vitamin C (ascorbic acid) tablet on the stain. If it lightens, it's a metal stain. If nothing changes, it's probably organic (leaves, algae) and will respond to chlorine.

What each color means

• Green or brown where leaves collect: organic stain. Brush with chlorine in the ideal range; it usually fades in a few days.
• Black spots that keep coming back in the same place, in a concrete pool: black algae. Brush daily with a stiff brush and keep chlorine high until it's gone.
• Reddish-brown or rust: iron, common with well water.
• Blue or blue-green: copper, which can come from copper-based algaecides or metal fittings.
• White crust at the waterline: calcium scale, when pH and hardness stay high for a long time.

How to treat them

• Metal stains: use a metal sequestrant, following the label, and keep the pH between 7.2 and 7.6.
• Calcium scale: correct the pH and use a scale remover, following the label.
• Organic stains: brushing, chlorine in the ideal range and the filter running.

In vinyl pools, use a soft brush and never let tablets or chlorine granules sit on the vinyl liner: they bleach and stain it. If the stain won't shift with the steps above, ask a professional before using stronger products.