Rust stains on the bottom or greenish-tinted walls aren’t just an aesthetic problem: they’re a sign that something isn’t working properly in the chemical balance or the metal components of your pool. The sooner you identify the cause, the easier (and cheaper) it is to fix.
In this guide we review the three most common issues related to pool water in Mallorca: rust stains, algae and general chemical imbalance.
Rust stains: why they appear
In Mallorca, it’s common to fill swimming pools with well water, which typically has a higher iron and manganese content than mains water. When this water comes into contact with chlorine, these metals oxidize and deposit as brown or reddish stains on the bottom and walls.
Other common causes:
- Corrosion of metal elements (ladders, screws, anchors) in direct contact with water
- Remnants of tools or metal objects forgotten at the bottom
- Unbalanced water pH, which accelerates the oxidation of the metals present
How to treat it: A treatment with a metal sequestrant prevents iron and manganese from precipitating as a stain. If the stain has already formed, a specific rust inhibitor applied locally will usually remove it without needing to drain the pool.
Algae: Identify the type before treating them
Not all algae are treated the same, and applying the wrong product only delays the solution.
Green algae: the most common type, giving the water a uniform green hue. They usually appear due to a lack of chlorine or insufficient filtration.
Black algae: These adhere to the walls as dark spots and are very resistant to standard chlorine. They require mechanical brushing in addition to specific chemical treatment.
Mustard algae (yellow): less frequent, they accumulate in shaded areas and are resistant to normal chlorine levels.
General treatment: first adjust the pH (ideally between 7.2 and 7.6), apply a shock treatment with chlorine and, in the case of black or mustard algae, supplement with a specific algaecide and manual brushing of the affected areas.
Water quality: the parameters that need to be monitored
Good water quality depends on more than just chlorine levels. Key parameters include:
pH: between 7.2 and 7.6. Outside this range, chlorine loses effectiveness and increases the risk of irritation or corrosion.
Total alkalinity: acts as a pH stabilizer; if it is unbalanced, the pH fluctuates constantly no matter how much it is adjusted.
Calcium hardness: water that is too hard promotes limescale buildup; water that is too soft causes corrosion of metals and linings.
Chlorine or salt level (depending on the system): should be kept constant, not just corrected when there is already a visible problem.
Prevention: the key is regularity
Most of these problems don’t appear overnight—they’re the result of weeks without checking the water’s chemical balance. Regular monitoring (weekly during peak season) detects deviations before they develop into staining, algae, or cloudy water.
Frequently Asked Questions
Do rust stains damage pool liners?
If left untreated, they can set into the liner and become more difficult to remove over time, although they don’t usually damage the pool structure itself.
Can I take a bath while treating algae?
It’s not recommended until the shock treatment has lowered the chlorine level to a safe range (usually 24-48 hours later).
How often should I check the pH of the water?
At least once a week during peak season, and after any heavy rain or significant pool use.
Does well water always cause rust problems?
Not always, but it is more likely if it has a high iron or manganese content. Testing the source water helps anticipate the necessary treatment.
Does your pool have rust stains, algae, or is the water not maintaining good condition despite regular treatment? At JM Cifre Piscinas, we diagnose and solve water quality problems in Mallorca.