How to increase pH in pool water, or lower it

Illustration.

Why pH matters

HealthyWA explains that chlorine works best when pH is between 7.2 and 7.8, with 7.5 ideal for swimmers. Above that range chlorine loses strength and calcium can cloud the water or scale surfaces; below it the water can sting eyes and corrode metal fittings. Heavy rain, lots of swimmers and the chemicals you add all move pH.

To lower pH (most common)

1. Test and note the reading.
2. Add pool acid (hydrochloric acid, or the dry acid sodium bisulphate) in a small dose, following the label for your litres. With the pump running, pour it slowly into the deep end, away from the skimmer.
3. Wait a few hours with the pump running, then retest. Repeat in small steps rather than one big dose.

Salt pools tend to drift up in pH, so regular small acid doses are normal.

To raise pH

Add soda ash (sodium carbonate) as the label directs, pre-dissolved in a bucket of pool water, then retest. If total alkalinity is also low, sodium bicarbonate (buffer) raises alkalinity with a smaller effect on pH.

Maple's tip

Never add acid and chlorine at the same time or into the same bucket. Add one, let the pump run, and add the other later.

The right pH range

Most guidance for backyard pools gives a pH range of about 7.2 to 7.8, with around 7.4 to 7.6 a comfortable target. Below the range, water becomes acidic and can corrode metal fittings, etch plaster and grout, and sting eyes. Above it, chlorine becomes much less effective, scale forms on surfaces and in the chlorinator cell, and the water can turn cloudy.

How to increase pH in a pool

To raise pH, add sodium carbonate (often sold as soda ash or pH increaser). Dissolve the dose in a bucket of pool water first if the label says to, then spread it around the deep end with the pump running. Add it in stages: a dose that is too large can overshoot and cloud the water. Wait several hours, or as the label says, with the pump running, then retest before adding more.

Sodium bicarbonate (bicarb, sold as buffer or alkalinity increaser) raises total alkalinity more than pH. It is the right product when both alkalinity and pH are low, because it raises alkalinity strongly and pH only a little.

Aeration also raises pH without chemicals. Running water features, pointing the return jets upwards to ripple the surface, or running a spa jet releases carbon dioxide and nudges pH up gently over a day or two. It is useful when pH is only slightly low and alkalinity is fine.

How to lower pH

Most pools, especially salt pools, see pH rise over time, so lowering it is the more common job. Add hydrochloric acid (pool acid) or dry acid (sodium bisulphate), diluted and added carefully as the label directs, into the deep end with the pump running. Always add acid to water, never water to acid. Wear eye protection and gloves, and store acid away from chlorine.

Total alkalinity: the buffer

Total alkalinity acts as a buffer that stops pH swinging wildly. If alkalinity is too low, pH bounces around and is hard to control; if it is too high, pH keeps creeping up and is hard to bring down. Many pools aim for an alkalinity of about 80 to 120 mg/L, though the right figure depends on the pool type and sanitiser; follow your test kit's or pool shop's targets. Fix alkalinity first, then fine-tune pH.

Why pH keeps drifting

• Salt chlorinators make the water slightly alkaline as they work, so pH rises steadily; many salt pools need acid every week or two.
• New plaster and concrete pools release alkaline compounds for months after construction.
• Rain is often slightly acidic and can lower pH, as can heavy use.
• Some chlorine products raise or lower pH as a side effect: liquid chlorine raises it, trichlor tablets lower it.
• Aeration from water features and spa jets raises it.

Testing pH accurately

Test at elbow depth, away from return jets, at the same time of day. Use fresh reagents or strips stored out of the heat, and read the colour in good light. Very high chlorine levels can affect some pH test results, so if a reading looks odd after shocking, test again the next day.

pH and comfort

Swimmers notice pH that is far out of range: red, stinging eyes and itchy skin are often blamed on chlorine but come from pH that is too high or too low. Keeping pH in the middle of the range makes the pool more comfortable and helps chlorine do its job with less of it.

Adding chemicals in the right order

When several things are out of range, fix them in order: first total alkalinity, because it controls how pH behaves; then pH; then free chlorine; and stabiliser and calcium hardness as needed. Making one change at a time, with the pump running and time to mix in between, avoids overshooting and wasted chemicals. Test after each step.

Doses for a typical pool

The dose needed to change pH depends on the pool's volume, its total alkalinity and the product's strength, so labels give doses per 10,000 litres for a typical change. Start with half the label dose for your volume, wait, retest and add more if needed. Small, staged doses are safer and cheaper than one large dose that overshoots. Our pool volume calculator gives your litres, and the pool chemical calculator helps with chlorine and stabiliser.

pH in salt pools

Salt chlorinators push pH up as they make chlorine, so salt pool owners usually spend more time lowering pH than raising it. Some chlorinators include an automatic acid dosing system that keeps pH in range; if yours does, check the acid container regularly and calibrate the pH probe as the maker says. Without one, test pH at least weekly in summer and add acid as needed.

pH and new pools

A new concrete or pebble pool releases alkaline compounds from the surface for many months, raising pH quickly. Pool builders usually give a start-up routine with frequent testing and acid additions during this time; follow it closely, because high pH in a new pool can cause scale and roughen the surface.

Common mistakes

1. Adding pH increaser and acid on the same day, chasing the number up and down.
2. Adding a full dose without retesting, and overshooting.
3. Ignoring total alkalinity, so pH never stays put.
4. Adding acid into the skimmer, where concentrated acid can damage equipment.
5. Testing right next to a return jet, where freshly added chemicals give a misleading reading.

Quick reference

• pH too low (below 7.2): add pH increaser (soda ash); if alkalinity is also low, use buffer (bicarb).
• pH too high (above 7.8): add pool acid or dry acid in small doses.
• pH keeps rising: check total alkalinity and lower it if high; expect this in salt pools.
• pH keeps falling: check total alkalinity and raise it if low; check whether you use trichlor tablets.

pH and different pool surfaces

Concrete, pebble and tiled pools are the most sensitive to pH. Low pH etches the surface and grout and can loosen pebbles; high pH leaves scale that roughens the surface and stains it white. Fibreglass and vinyl pools are less affected by etching but still suffer from scale and from discomfort for swimmers when pH is out of range. Whatever the surface, keeping pH steady in the middle of the range protects it.

pH and metal fittings

Low pH water attacks copper heat exchangers in heaters, ladder rails and other metal fittings, which can release copper into the water and stain surfaces or turn hair green. If you have a heater, keeping pH from dropping too low is especially important.

Testing tools

Test strips are quick and convenient; liquid drop kits are often more precise; digital testers and smart pool monitors give readings without colour matching but need calibration. Whatever you use, keep it out of heat and sunlight, replace reagents by their expiry date, and compare with a pool shop test now and then.

How often to test pH

Test pH at least twice a week in summer, after heavy use or rain, and after adding chemicals. In winter, once a week is usually enough. Salt pools and new pools need more frequent checks.

pH and chlorine strength

The reason pH matters so much is that chlorine's strength depends on it. At the low end of the range, more of the chlorine is in its most active form; as pH rises, less of it is, so the same chlorine reading kills germs and algae more slowly. Keeping pH in the middle of the range lets you run a sensible chlorine level and still have a clean, safe pool.

pH and swimmer comfort

Human tears have a pH a little above 7, which is part of why pool pH around 7.4 to 7.6 feels comfortable. Very high or very low pH irritates eyes and dries skin and hair, and swimmers often blame chlorine for it.

Our short answer

Keep pH between 7.2 and 7.8, ideally around 7.5. To increase pH, add soda ash in small doses; if alkalinity is also low, use bicarb. To lower it, add acid carefully. Fix alkalinity first, test after every change, and expect salt pools to need acid regularly.

pH and the seasons

In summer, heat, sun and swimmers push chlorine use up, and salt chlorinators run harder, so pH in salt pools climbs faster. In winter, changes are slower and weekly testing is usually enough. Heavy rain in any season can lower pH and alkalinity, so test after storms. Keeping a simple log of readings over a year shows your pool's own pattern and makes adjustments easier to predict.

When to ask a pool shop

If pH will not stay in range despite correct alkalinity, if readings swing widely from day to day, or if you are unsure which product to use, take a water sample to a pool shop for a full test and a dosing plan.

A quick pH routine

Twice a week in summer, test pH at elbow depth in the evening. If it is above the range, add a small measured dose of acid with the pump running; if below, add pH increaser. Retest the next evening. Once a month, test total alkalinity and correct it first if it is out of range. This simple routine keeps most pools in balance through the season without large swings or expensive corrections.

pH increaser versus buffer: which to buy

Buy soda ash (pH increaser) when pH is low but total alkalinity is in range. Buy bicarb (buffer or alkalinity increaser) when both are low, or when alkalinity alone is low. Many pool shops sell both; check the active ingredient on the label, sodium carbonate or sodium bicarbonate, rather than relying on the brand name, because names vary between brands.

Common questions