Understanding your pool's water balance

pH must stay between 7.2 and 7.6. Below that, your water corrodes equipment and stings your eyes. Above it, chlorine loses its effectiveness and limescale deposits everywhere — a real hassle in the Var, where mains water is already hard.

Measure it first, adjust it first. Everything else follows from this reading.

Active free chlorine must stay between 1 and 3 mg/L. Below that, bacteria and algae move in. Above it, it's uncomfortable to swim in.

Two tips:

• Chlorine tablets in the skimmers for everyday upkeep.
• Shock treatment (granules) after a storm, heavy use, or more than 3 days without filtration.

Alkalinity should stay between 80 and 120 mg/L. It's what stabilises your pH. If your pH won't hold steady between readings, it's almost always alkalinity that's too low. Raise it, and pH will settle on its own.

Ideal is between 150 and 250 mg/L of limescale. In the Var, it's often above 300. Beyond that, you'll see white deposits on the walls and the equipment ages faster.

A limescale sequestrant once a month is enough to neutralise the damage.

Stabiliser (cyanuric acid) protects chlorine from the sun. Without it, your chlorine burns off within hours. With too much, it actually blocks chlorine's action: it's the hidden cause behind most water that turns in high summer.

Ideal is between 30 and 50 mg/L. Above 75, the only fix is a partial drain of the pool. If you've never measured yours, it's urgent.