Green or Cloudy Pool Water: Fixing It Fast in a Mallorca Heatwave

Clear on Monday, green soup by Friday. On Mallorca, pool water turning green after a hot week rarely has much to do with poor care. At a water temperature of 30°C (86°F) and the UV intensity of an August day, free chlorine gets used up within hours, and algae multiply in warm water faster than a normal dosing schedule can keep up with. Add chemicals in the wrong order at that point and you burn through money while making the water murkier instead of clearer. Here is how to read the color to find the cause, the order to bring the pool back in, and what it costs in chemicals and pool service. What else a pool costs over a year is covered in the guide to running costs on Mallorca.

Green, milky, or brown: the color tells you the cause

Before you dose anything, look at the water in daylight and run your hand along the pool wall.

• Green, and the wall feels slippery: that is algae. Free chlorine is at zero, usually for two to three days already. The darker the green, the more chlorine you need.
• Milky and cloudy, but the wall is smooth: usually a pH above 7.8, often after topping up heavily with the island's hard tap water, or a sand filter that has not been backwashed in weeks. Dead algae after a shock chlorination can also look like this, which is actually a good sign.
• Brown, or greenish and clear without any coating: iron or manganese, typical for fincas on a private well. Chlorine oxidizes the metals and the water discolors; a metal sequestrant pulls them back out.
• Black spots in the grout: black algae, the toughest kind. Brushing, a high chlorine dose, and patience, often over several rounds.

A simple test helps sort out which one you have: hold a glass of pool water in front of a sheet of white paper. A green tint points to algae growth, a whitish haze to particle turbidity.

Why pools turn on Mallorca within hours

Free chlorine without a stabilizer breaks down under UV radiation - on a clear summer day you can lose most of your dose in two to three hours. Cyanuric acid as a stabilizer slows this down, but only while it stays between 30 and 50 mg/l. If you run stabilized chlorine tablets all summer, cyanuric acid builds up. Above 100 mg/l, chlorine becomes so sluggish that a measured reading of 1.5 mg/l still cannot hold back algae. That is the classic island trap: the tester shows chlorine, and the pool is green anyway.

Then there is water temperature. Above 28°C (82°F), chlorine demand rises noticeably, because disinfection reactions speed up and algae divide much faster in warm water. A pool that got by on one tablet a week in May can need twice that in August, plus regular re-testing. And once swimmers, sunscreen, and a Sahara dust day all land on the same weekend, that is all it takes.

Spain's Real Decreto 742/2013 sets the benchmark for water values, requiring a pH between 7.2 and 8.0 and free chlorine between 0.5 and 2.0 mg/l for public pools. It is not binding for your private pool, but pool companies on Mallorca use it as their reference. When rescuing a pool that has turned, aim deliberately for the lower end of the pH range, because that is where chlorine works hardest.

The fix, in the right order

A shock chlorination is a single, sharply raised dose of chlorine - usually five to ten times the normal maintenance dose - added to oxidize algae, bacteria, and combined chlorine in one pass. It only works if the pH is right beforehand and the pump keeps running afterward.

Salt chlorination pools turn differently

Owners with a salt system often assume they are immune to green water. But the cell only produces chlorine while it is running, and the run time you set in June is often no longer enough by August. Add the island's hard water: the electrode plates scale up and output drops without triggering any error message. Test free chlorine with a hand tester weekly in summer - do not rely on the unit's own readout alone.

If a salt pool turns, the boost function on its own rarely fixes it. You still need a classic shock chlorination with granules or liquid chlorine, plus a check of the salt level - after heavy topping-up it is often below the cell's minimum. Descale the cell in diluted acid if needed. For which system makes sense on the island and what the normal values should look like, see the post on pool water treatment on Mallorca.

What chemicals and pool service cost

For a typical private pool of 40 to 60 cubic meters, a DIY rescue stays within a manageable budget. Prices below are typical for hardware stores and pool shops on the island; branded products cost more.

In July and August, the emergency call-out is a timing problem more than a price problem. Companies are booked out with contract customers, a one-off job often waits a week, and the pool does not get better while it waits. If you are not on the island year-round, a maintenance contract over the summer usually works out cheaper than two rescue jobs. What the pump draws over 48 hours of continuous running and what water costs on the island is covered in the guide to electricity and water on Mallorca.

Draining the pool is not an option on Mallorca

The instinct to simply pump out a dark green pool and refill it is wrong on the island for several reasons. First, water is scarce: according to the Balearic water authority's drought index, Mallorca's reserves stood at 39 percent in July 2026, with three supply units (Es Pla, Manacor-Felanitx, and Migjorn) in the Alerta tier. At that tier, municipalities can restrict pool filling. Manacor issued water-saving recommendations for gardens and pools in late August 2026 and warned that temporary supply cuts could follow if reserves keep falling. Second, refilling a 50-cubic-meter pool with tap water easily costs over 100 EUR depending on the municipality and tariff tier, and much more with a tanker truck. Third, an empty pool in summer is a structural risk: with a high water table or wet ground, the shell can float up, and liner pools shrink in the heat.

Even a deep green pool can usually be rescued with two rounds of shock chlorination and a few days of filter run time. A partial water change of 20 to 30 percent only makes sense once cyanuric acid is above 100 mg/l, because it cannot be broken down chemically. What rules currently apply to refilling in your municipality is covered in the post on filling your pool during water shortages.

How long does it take for a green pool to clear up again?

For light green water, a pH correction, a shock chlorination, and continuous pump running usually clear it in 24 to 48 hours. Dark green pools need two rounds and three to five days, because the filter only catches the dead algae after several backwash cycles.

Can I still swim in green pool water?

No. Green water means there is no free chlorine left, which also means no disinfection against bacteria. After a shock chlorination the opposite applies: only get back in once free chlorine has dropped below 3 mg/l.

Why is my pool water green even though the tester shows chlorine?

Almost always because of excess cyanuric acid. Above 100 mg/l it binds chlorine so tightly that it stops killing algae even though a test still shows a reading. Test cyanuric acid and, if needed, replace part of the water.

Conclusion

Green pool water is more the rule than the exception on Mallorca in high summer, and it is fixable within two days if you keep the order: brush, bring pH to 7.0-7.4, shock-chlorinate after sunset, run the pump, flocculate, backwash. The chemicals cost 40 to 80 EUR, an emergency call-out from a pool company 80 to 150 EUR plus material, and in August it is usually the appointment slot that is scarce, not the money. Test free chlorine and cyanuric acid weekly in summer and raise the dose ahead of forecast heatwaves, and you will rarely need the rescue routine. With reserves at 39 percent and three regions in the Alerta tier, draining remains the worst option of all. For a full overview of the ongoing costs around a house and pool, see the guide to running and operating costs on Mallorca.