Pool Water Balance on the Highveld: pH, Alkalinity, Calcium

Clear water is not the same as balanced water. A pool can look fine while its water is slowly etching the marbelite, scaling up the salt cell or eating into the heat pump's heat exchanger. On the Highveld, hard municipal water, strong sun, summer thunderstorms and dry winters all push the chemistry around, so balance takes a little more attention here than in many other places.

Our complete guide to pool maintenance in Centurion covers the weekly basics. This article goes deeper into the balance itself: what each reading means, the order to adjust them in, and how to handle the chemicals safely.

What balanced water means

Water is always trying to reach equilibrium with what it touches. If it is short of calcium or too acidic, it takes what it wants from the pool surface, grout and metal fittings. That is corrosive water. If it carries too much calcium at too high a pH, it deposits the excess as scale on tiles, surfaces and inside equipment. Balanced water sits between the two, so it neither dissolves your pool nor coats it.

The reference ranges

These are the commonly used ranges for a residential pool:

  • Free chlorine: 1–3 ppm
  • pH: 7.2–7.6
  • Total alkalinity: 80–120 ppm
  • Calcium hardness: 200–400 ppm
  • Stabiliser (cyanuric acid): 30–50 ppm. For salt pools, follow your chlorinator's manual.
  • Salt level (salt pools only): check your chlorinator's manual, because each model has its own range.

Adjust in this order

The readings affect one another, so the order matters. Adjusting them in the wrong order means chasing numbers that keep moving.

  • Total alkalinity first, because it holds pH steady.
  • pH second, once alkalinity is in range and pH will stay where you put it.
  • Calcium hardness third, which changes slowly and matters most for surfaces and equipment.
  • Chlorine last, because how well chlorine works depends on the pH.

Total alkalinity: the buffer

Total alkalinity is the water's ability to resist changes in pH. When it is low, pH swings about with every rainstorm and every dose of chlorine. When it is high, pH drifts upwards and is hard to bring down.

To raise alkalinity, sodium bicarbonate (bicarb) is the usual product. To lower it, acid is added gradually over time, with the pump running and repeated tests in between. Lowering alkalinity is slow work, which is another reason not to overshoot when raising it.

pH: how well everything else works

pH measures how acidic or basic the water is. Above 7.6, chlorine loses much of its strength, calcium starts to come out as scale and cloudiness, and swimmers may notice itchy skin. Below 7.2, the water becomes aggressive: eyes sting, metal corrodes and surfaces etch.

Pools with salt chlorinators tend to push pH upwards, because of the way the cell makes chlorine, so they usually need regular acid additions. To lower pH, pool acid or dry acid is used. To raise it, soda ash is the usual product.

Calcium hardness: the surface protector

Calcium hardness is the amount of dissolved calcium in the water. Below about 200 ppm, the water draws calcium out of marbelite and grout, leaving the surface rough, pitted and prone to staining. Above about 400 ppm, particularly with high pH, calcium deposits as scale.

On the Highveld the more common problem is too much calcium rather than too little, for reasons explained below.

Chlorine: last, but checked most often

Once alkalinity, pH and calcium are right, chlorine can do its job. Free chlorine should sit at 1–3 ppm. It is used up by sunlight, swimmers, leaves and rain, so it is the reading that changes fastest and needs checking most often. Our guide to pool chlorine types explained compares the common forms of pool chlorine and what each does to your water balance.

Stabiliser: useful, up to a point

Strong Highveld sun breaks down unprotected chlorine very quickly. Stabiliser, also called cyanuric acid, shields chlorine from ultraviolet light so it lasts longer in an outdoor pool.

The catch is that stabiliser does not evaporate or get used up. It only leaves with water that is drained, backwashed or splashed out. Stabilised chlorine products, such as many tablets and some granules, add a little stabiliser with every dose, so the level creeps up over months. Too much stabiliser makes chlorine sluggish: the test shows chlorine is present, but it is slow to kill algae and bacteria. The only practical way to bring it down is to replace some of the water with fresh water.

Test stabiliser every month or two, and if it keeps rising, consider a chlorine product that does not contain it.

Hard municipal water and scale

Much of the municipal water in Centurion and Pretoria is hard. Every top-up adds calcium, and every hot, dry day evaporates water while leaving the calcium behind. Over a year or two, calcium hardness can climb well past the top of the range.

The signs of scale are easy to spot once you know them:

  • A white or grey crust at the waterline on tiles
  • A rough, sandpapery feel on steps and surfaces
  • White deposits on the plates of a salt chlorinator cell, which reduce its output
  • Reduced performance from a heat pump as scale builds up inside its heat exchanger

Keeping pH at the lower end of its range slows scale formation considerably. Scale that has already built up at the waterline can be removed: tile descaling is available as an add-on to a pool clean, at R800.

Corrosive water: low pH and low calcium

The opposite problem is just as damaging and less visible. Water with low pH, low alkalinity or low calcium attacks what it touches.

  • Marbelite and plaster become etched, rough and blotchy, and lose their smooth finish.
  • Heat exchangers in heat pumps can be damaged by aggressive water over time.
  • Metal fittings, ladders, light rims and pump parts corrode, and the metal can stain the pool surface.

Summer rain can contribute. Rainwater is soft and slightly acidic, so a big storm dilutes calcium and alkalinity along with the chlorine.

If the surface is badly stained or scaled beyond what brushing and balanced water can fix, an acid wash removes a thin layer of the surface to expose clean material underneath. Our acid wash service is R3,500 for a full acid wash. Afterwards, keeping the water balanced is what stops the problem returning.

Test strips or drop kits?

Test strips are quick and convenient. Dip, wait and compare the colours. They are good for a regular check of chlorine and pH, but they are less precise, and strips that have been left in a damp pump room or are past their date give inaccurate readings.

Drop kits, also called titration kits, are more precise, especially for alkalinity and calcium, where you count the drops it takes to change the colour. They take a little longer and a steady hand.

Whichever you use, take the sample from elbow depth, away from the return jets and the weir, and keep the kit out of direct sun. Replace reagents and strips when they expire.

Adding acid safely

Pool acid is corrosive and must be handled with care.

  • Read the label and follow the manufacturer's instructions for dose and handling.
  • Wear protective gear: chemical-resistant gloves, eye protection and old clothes.
  • Always add acid to water, never water to acid. If you are diluting it, fill a plastic bucket with pool water first and pour the acid slowly into the water. Adding water to acid can make it spit violently.
  • Keep the pump running so the acid is mixed through the pool.
  • Pour slowly in front of a return jet in the deep end, away from the walls, steps and fittings.
  • Add in portions. Let the water circulate for a few hours and retest before adding more.
  • Never mix chemicals. Acid and chlorine together release a toxic gas. Use separate buckets and scoops, and never store them together.
  • Store safely: in the original container, in a cool, shaded, ventilated place, out of reach of children and pets.

Rain and top-ups

After a heavy summer storm, test the water. Expect chlorine to have dropped and alkalinity and pH to have shifted. If the water level has risen above the weir opening, lower it by running the filter to waste so the weir can skim properly again.

In the dry winter months, the opposite happens. Evaporation lowers the level, you top up with hard municipal water, and calcium creeps upwards. Test calcium hardness every few months so you notice the trend before scale appears.

Getting help

If your readings keep drifting, or you are not sure which to correct first, a weekly pool maintenance plan takes it off your hands: we test the water on every visit and correct the balance based on the readings. For advice on a specific problem, send us a WhatsApp on 081 577 5051 with your test readings and a photo of the pool.

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