Pool water balance: why it slips faster on Mallorca

Most of the pool problems that reach us in summer are not pool problems. They are pH problems that nobody measured for a fortnight.

Once the relationships are clear, a pool needs far less chemistry – and stops swinging between cloudy and over-chlorinated.

The four readings that matter

Everything else follows from these. They describe the state of the water:

• pH: target range 7.0 to 7.4. It decides how well everything else works.
• Free chlorine: in a private pool usually 0.5 to 1.5 mg/l. More is not safer, only less pleasant.
• Total alkalinity: 80 to 120 mg/l. This is the buffer that keeps pH where you put it.
• Calcium hardness: 200 to 400 mg/l. Too little attacks grout and render, too much settles as scale.

Guide values for a privately used pool. Saltwater chlorination, active oxygen systems and pools in public use may call for different targets – when in doubt, follow the equipment manufacturer's specification.

Why pH comes first

Chlorine does not work independently of pH; it works according to pH. Above 7.6, the same amount of chlorine loses a substantial part of its disinfecting power. The test kit still shows a respectable chlorine reading – far less is happening in the water than the number suggests.

The usual response is to add more chlorine. The result is a pool that smells of chlorine, stings the eyes and still looks cloudy. That smell, incidentally, does not come from chlorine itself but from chloramines – the compounds formed once chlorine has already done its work.

So the order is: alkalinity first, then pH, then disinfection. Correct pH without checking the buffer and you will be correcting it again in three days.

What makes the water here different

Mains and well water on the island is hard. Scale raises pH and brings calcium with it, which settles at the waterline, in heat exchangers and on saltwater chlorinator cells.

Then there is evaporation. What evaporates is pure water – everything dissolved stays behind. Across a hot summer the water concentrates measurably, especially where it is topped up regularly with hard water. A pool that was perfectly balanced in June can read quite differently in August without anyone having done anything wrong.

If you top up from your own well, have that water analysed once. Iron and manganese in the fill water are the most common cause of stains that later have to be worked out of the surface.

Sun, stabiliser and a common trap

UV light destroys free chlorine – very quickly in a Mallorcan high summer. Cyanuric acid, the stabiliser already present in many chlorine tablets, protects against that.

And there lies the trap. Stabiliser does not evaporate and is not consumed. It accumulates with every tablet. Below roughly 50 mg/l it protects the chlorine; beyond roughly 75 mg/l it noticeably slows it down. The pool then has enough chlorine on paper and algae in reality.

In practice, cyanuric acid can only be reduced by replacing water. That is why it is worth measuring once a season rather than letting it build up unnoticed over years.

Calima, pollen and sun cream

Saharan dust does more than settle visibly on the surface: it carries nutrients into the water and consumes chlorine. After a calima episode a check is worthwhile even if the pool already looks clear again.

The same is true of pollen in spring and of sun creams and body oils in high summer. Both bind disinfectant without anyone being at fault.

How often to test

In peak season and with regular use, twice a week is a sensible rhythm; with heavy use, more. In winter less often is fine – but not never.

Regularity matters more than frequency. Two readings a week at a similar time of day show a trend. Ten readings in July and none in August show nothing at all.

Write the values down. A single reading tells you how the water is. A series tells you where it is going – and that is the information you can act on before anything goes wrong.

Signs that something is off

• The water is slightly milky although chlorine reads correctly – suspect high pH or an overloaded filter.
• A strong chlorine smell and stinging eyes – suspect chloramines, not excess chlorine.
• A green film in shaded corners – suspect too little effective chlorine, or cyanuric acid that is too high.
• Rough white deposits at the waterline – scale, usually together with high pH.
• Chlorine consumption rises for no obvious reason – often organic load, or a demand nobody has looked for yet.

If the pool has already turned

Work back in the same order: test, set alkalinity and pH, run the filter, and only then disinfect. Starting with a shock treatment while pH sits at 8 uses a great deal of product for very little effect.

Add chemicals one at a time and never together in a bucket – some combinations react violently. And always product into water, never water onto product.