Rust stains in the pool: where they come from and what helps

Brown, rust-red or greyish marks on the floor and walls are irritating because they make a well-kept pool look neglected. And because the obvious response – chlorinate hard – makes many of them worse.

So the first step is not a product. It is a diagnosis.

The vitamin C test: one minute for a diagnosis

Metal stains and organic stains look almost identical but need opposite treatments. This test separates them:

• Take a vitamin C tablet (ascorbic acid) and press or rub it directly onto a stain.
• If the stain lightens noticeably within a minute, it is a metal stain.
• If nothing happens, try the same spot with a chlorine tablet. If it lightens now, it is an organic stain – leaves, algae, fruit, insects.
• On vertical walls, put the tablet in a sock and hold it against the spot on a telescopic pole.

The test also settles the most common mix-up. Shock chlorinating a metal stain oxidises the metal further and fixes the discolouration. The stain gets darker, not lighter.

Where the metal comes from

Metal almost always arrives by one of three routes:

• Through the fill water. Well water on Mallorca often contains iron and manganese. While it stays dissolved you see nothing. As soon as chlorine meets it, it oxidises out and settles.
• From corroding parts. Screws, clips, ladders, handrails, heat exchangers and, in older installations, pipe fittings. A single rusting fixing is enough for a distinct plume.
• From outside. A garden tool, a tin, a furniture foot – in the worst case reinforcement lying too close to the surface where the concrete cover has cracked.

Why it happens more often here

Two things coincide on Mallorca. First, top-up water often comes from a private well, and hardly anyone has seen an analysis of it. Second, a great deal of water evaporates over the summer and is continuously replaced. Every top-up brings new metal that stays in the pool.

Add salt-laden air near the coast, which accelerates corrosion of fittings and equipment. Saltwater chlorination systems are particularly sensitive, because the water itself is more aggressive towards unsuitable metal parts.

Dealing with a metal stain

The order matters, otherwise the stain returns:

• Find the source and stop it. While it is still running, any treatment is cosmetic.
• Have the fill water analysed if you top up from a well. A pre-filter on the fill line solves the problem more permanently than any treatment.
• Bind the metal in the water with a sequestrant, which holds the ions in solution so they cannot settle again.
• Only then treat the stains and filter thoroughly. Once bound, the metal has to leave the water – otherwise it drops out again at the next chlorine peak.
• Keep chlorine low during this phase. Oxidation is precisely what you do not want.

Dealing with an organic stain

Here it is simple in the opposite direction: brush, raise chlorine, filter. Organic stains break down through oxidation.

If they keep coming back, look up rather than down. Overhanging figs, oleander, bougainvillea or palm fronds above the water are the usual cause, and pruning is a more lasting solution than any treatment.

Prevention is markedly cheaper

• Have well water analysed once and, if needed, feed it through a pre-filter.
• Keep pH steady between 7.0 and 7.4 – metals drop out more readily when pH swings.
• Keep iron and unknown metals out of and away from the pool.
• Inspect fittings, ladders and heat exchangers regularly for early corrosion.
• Test the water after any large top-up rather than waiting for the next routine visit.

With stubborn or widespread discolouration on render or tiles, establish what the surface will tolerate before any acid treatment. An unsuitable treatment costs the finish – which is far more expensive than the stain.