The Low pH Danger Zone: Bob Lowry Breaks It Down

Low pH in pools doesn’t just mean “a little acidic.” It means water that actively looks for things to dissolve. Over time that turns smooth plaster rough with etching, chews through heater headers and pump impellers, and releases metals into the water that later stain surfaces. Many pros are trained to fight high pH, but the hidden damage from low pH is often worse and more expensive. Vinyl and fiberglass pools are especially vulnerable because their surfaces don’t release buffering minerals the way plaster does. When alkalinity dips and pH slides, aggressive water pulls from metals first, and those dissolved metals remain invisible until they hit saturation and precipitate into brown or green stains.

The cure starts with better data. A cheap test kit can wreck a service route: wrong readings cause wrong doses, which trigger more corrosion, cloudiness, and callbacks. Digital photometers that test copper and iron give you early warning before stains form, and they repay themselves when a single avoided heater repair offsets the cost. Test at every visit and add periodic copper and iron checks, especially on routes with older equipment or source water known for metals. If results look off, verify with a second method rather than chasing numbers with chemicals.

Understanding surface chemistry prevents repeat problems. Plaster contributes a trickle of calcium and alkalinity that nudges balance toward stable, but vinyl and fiberglass offer no mineral buffer. Add acidic chlorine like trichlor in feeders or tablets, and you can push pH lower week by week, especially in lightly used bodies of water without aeration. In hot tubs, the story splits: turbulence often raises pH, but heavy dosing with trichlor or dichlor can push it down, and small volumes magnify every mistake. Balance targets are practical, not theoretical—aim for comfort and stability first, then polish with fine adjustments.

When pH is low, choose the right tool. Baking soda (sodium bicarbonate) primarily raises alkalinity and has a ceiling pH near 8.3, so it barely nudges pH unless the water is very acidic. Soda ash (sodium carbonate) has a high pH and efficiently lifts both pH and alkalinity, making it ideal when both numbers are low. Borax raises pH with a moderate lift and minimal alkalinity change, useful for targeted corrections. For spas with pH near 6 and alkalinity around 40 ppm, bring alkalinity to 80–90 ppm with baking soda, then aerate with jets and blowers to raise pH into the 7.4–7.6 range within minutes. Aeration drives off carbon dioxide and increases pH without altering alkalinity, a controlled way to finish the balance.

Borates add another layer of stability. At 30–50 ppm, they buffer against pH rise and can improve water feel, but they also require an alkalinity interpretation adjustment when you calculate the saturation index. The effect grows as pH rises, so read your numbers in context: at pH 7.5, 50 ppm borate slightly skews total alkalinity; at higher pH, the correction increases. Keep notes on each pool’s borate level to avoid overcorrecting or misreading stability. Meanwhile, remember that soda ash increases alkalinity ounce for ounce more than bicarb, useful when you want to lift both metrics quickly.

Finally, revisit chlorine myths through the lens of cyanuric acid. Textbooks once taught that higher pH kills chlorine power by shifting the HOCl/OCl- balance. With CYA in the water, most chlorine is bound, and the simple percentage chart isn’t the full story. What matters more is maintaining sufficient free chlorine relative to CYA to prevent algae and oxidation shortfalls. Don’t obsess over 7.4 versus 7.6 for kill speed; focus on balanced water that keeps bathers comfortable and equipment safe, with pH centered near 7.5 to avoid eye and skin irritation that climbs above 7.8. With accurate testing, informed chemical choices, and attention to metals, you protect surfaces, keep swimmers happy, and turn chemistry from guesswork into a repeatable system.