Water hardness in pools and spas: How to achieve the perfect balance

Water hardness is one of those “silent” parameters that, when it’s right, no one notices… but when it’s out of balance, your pool or spa screams about it with lime scale, cloudy water, damaged equipment, and uncomfortable bathers.

The good news is that, with correct measurement and well-planned adjustments, you can keep the water at its ideal level and extend the life of your facility.

What is water hardness?

Water hardness indicates the amount of dissolved minerals, mainly calcium and magnesium. In pools and spas, we are most interested in calcium hardness, because it directly influences how water behaves with surfaces and equipment.

• Low hardness: the water tends to “seek out” minerals and can become more aggressive with materials and components.
• High hardness: excess calcium promotes scaling (lime) and can cloud the water.

Why water hardness affects bathing comfort

When hardness is out of range, it’s not only noticeable visually: you can feel it too.

• If it is too high, the water can feel “rough,” leave a film on the skin, and cause dryness. It is also common to see limescale deposits on the waterline, nozzles, or decorative elements such as waterfalls.
• If it is too low, the water can feel “flat” and, due to associated imbalances, end up affecting finishes and joints over time.

And here’s a key nuance: many problems that seem to be “just lime” are exacerbated when other parameters are not in balance. That’s why it’s always a good idea to check the overall balance, starting with the pH level, before making quick corrections.

Differences between the hardness of indoor and outdoor pool water

Outdoor pools tend to experience more “roller coaster” changes:

• Evaporation and refilling: heat causes water to evaporate, but calcium remains, increasing hardness.
• Frequent refilling: if you refill with hard water, the hardness increases every week without you noticing.
• Rainfall: this can temporarily dilute the water, but evaporation then causes it to become concentrated again.
• Temperature and sun: these accelerate reactions and lime precipitation.

Indoors, as there is less evaporation and fewer external inputs, water hardness tends to be more stable, although indoor spas (due to temperature and agitation) can precipitate lime faster if the hardness is too high.

How can I find out the hardness of my pool water?

The practical way is to measure it with:

• Drop kit (titration): this is usually the most reliable method in the field.
• Test strips: quick, useful for frequent monitoring (best to confirm if in doubt).
• Professional analysis: recommended for large pools, commercial spas, or if there are recurring problems.

Recommended water hardness table

The ranges may vary slightly depending on the type of pool, finishes, temperature, and water source, but as a general guide:

Tips for determining the hardness of your pool water

• Measure at least once a month (and weekly in spas or in summer if you refill often).
• Take an extra measurement if you have changed the water, there has been heavy rain, or you have refilled several times.
• Record the results (app, notebook, or Excel): you will see patterns and avoid impulsive corrections.
• Don’t adjust blindly: first confirm hardness, pH, and alkalinity. Many “lime” problems worsen if the pH is high.
• If the water is cloudy or there is lime, also measure the temperature and check the filtration time.

Common problems caused by incorrect water hardness

When water hardness is not balanced, the following problems typically occur:

• Limescale deposits (calcium) in water lines, nozzles, walls, and heating elements/heaters.
• Cloudy or “milky” water due to calcium precipitation.
• Loss of efficiency in heaters and exchangers (higher consumption and wear).
• Stains and film on tiles, porcelain, or rough surfaces.
• Corrosion or deterioration of components if the hardness is very low and the water becomes aggressive.
• Higher maintenance costs: more cleaning, more downtime, and more chemical adjustments.

How to adjust water hardness

If your measurements show that the hardness is outside the normal range, it is best to correct it gradually, avoiding sudden changes.

When the hardness is low, it is best to increase it gradually. Calculate the correction according to the volume of the pool or spa and the target, add a hardness increaser according to the manufacturer’s recommended dosage, and do so with the filtration system running so that the product is distributed evenly.

If the hardness is high, the most effective approach is usually to reduce the mineral load. This is usually done with a partial water change and, in more demanding situations, by considering reverse osmosis or external treatment.

To keep hardness stable throughout the season, it works well to combine a regular measurement plan with preventive maintenance of the chemical balance. And if you are looking to reduce your dependence on traditional treatment, you may be interested in exploring alternatives to chlorine as part of an overall strategy.