Pool stains: identify and remove every type (UK guide)

Identify before you treat

Touching the wrong chemical to the wrong stain can set it permanently. Chlorine on a metal stain locks it in; acid on an organic stain does nothing useful. And because most outdoor pools in Britain only run from May to September, a stain you spot at spring opening has often had all winter under the cover to bed in. Identify first, then treat.

The vitamin C test

Drop a 500 mg vitamin C tablet — any chemist sells them — onto the stain and hold it there for 30 seconds with a brush. If the stain lifts and leaves a lighter patch under the tablet, it is metal. If nothing happens, it is organic and chlorine will sort it.

This is the single most useful test in stain removal. Keep a tube in your pool kit.

Removing metal stains

1. Drop free chlorine below 1 ppm (mg/L). British sunshine takes its time burning chlorine off, so a chlorine neutraliser (sodium thiosulphate) is usually the quicker route
2. Add ascorbic acid at roughly 12 g per 1,000 litres — about 450 g for a typical 38,000-litre pool — straight into the water
3. Brush the stains. They lift within minutes
4. Add a metal sequestrant (HEDP-based) per the label to hold the dissolved iron in solution
5. Run the pump for 24 hours
6. Bring chlorine back up to 3 ppm slowly over 3 to 4 days

If you skip the sequestrant, the metal redeposits the moment chlorine returns and the stain comes straight back.

Removing organic stains

For organic stains (algae, leaves, pollen), chlorine is the answer. Brush the stain, raise free chlorine to shock level and hold it there for 24 hours.

For stubborn black algae, scrub with a pumice stone (concrete and tiled pools only, never a liner) or a stainless steel brush, then rest a chlorine tablet directly on the spot for an hour.

Pink slime

Pink slime is a bacterial biofilm that shrugs off normal chlorine levels. Treatment:

1. Clean the filter completely (it lives in there too)
2. Shock to 30 ppm chlorine
3. Hold shock level for 24 to 48 hours
4. Scrub every surface
5. Run the pump continuously

Stains in liner pools

A large share of British garden pools are liner pools, so take care. Pumice stones, stainless steel brushes and acid washing will all ruin a vinyl liner. Use nylon brushes and liner-safe products only. Ascorbic acid is liner-safe for metal stains; never let a chlorine tablet rest on the liner. For deep-set stains in an old liner, sometimes the only honest fix is a new liner.

Prevent stains from coming back

• Add a metal sequestrant once a season (iron, copper)
• If you fill or top up from a private borehole supply, pre-filter the water with a hose-end metal filter
• Avoid copper-based algaecides
• Keep pH in range — high pH precipitates metals fast, and the hard mains water across much of southern and eastern England pushes pH up on its own
• Keep an eye on heater corrosion. Nearly every outdoor pool in Britain is heated, and a leaking heat exchanger quietly dumps copper into the water

When to call in a professional

Call one for a full acid wash (concrete pools only — a nasty job, not a DIY project). Call if stains return within a week of treatment: you have a metal source you have not found yet. And call before attacking a vinyl liner with anything aggressive.

Stains cost bookings if you hire out your pool

Private pool hire is growing quickly across the UK, and guests choose with their eyes: a bright, stain-free pool wins bookings in London, Manchester or anywhere else, while a stained one gets scrolled past. If you have been thinking about earning from your pool, start with our guide to renting out your pool in the UK.

A note on money, plainly: prices on Pool Rental Near Me currently display in US dollars, and hosts are paid out in pounds sterling via Stripe. Founding hosts keep 100% of their price — 0% host fees, forever. A Saturday morning spent on stain removal pays for itself on the next booking.

Ready? List your pool and get your first hire on the calendar before the season slips by.