When pH drops too low in a pool, the water becomes acidic enough to irritate skin and eyes, corrode metal fixtures, and eat away at pool surfaces. The ideal range for a swimming pool is 7.2 to 7.6, and anything below 7.0 signals a problem that needs attention. Below 6.8, the water becomes aggressive enough to cause real damage to both swimmers and equipment.
What Low pH Does to Swimmers
Acidic pool water is the main driver behind the stinging, red eyes people blame on “too much chlorine.” The real culprit is usually pH that’s drifted too low. When pool water falls below 7.0, it becomes increasingly irritating to soft tissues. Your eyes are especially vulnerable because pool water has a lower salt concentration than your tears, which causes water to move into the cells of your eye and make them swell. Combined with the chemical irritation from the acidic water itself, this produces what’s known as swimmer’s eye: redness, burning, itching, and temporarily cloudy vision.
Your skin takes a hit too. Low-pH water strips away the natural oils that protect your skin, leaving it dry, tight, and itchy after swimming. Prolonged exposure can cause rashes. Hair becomes brittle and discolored over time, especially color-treated hair. If you or your kids consistently come out of the pool with irritated skin and burning eyes, testing the pH should be your first step.
Damage to Pool Surfaces and Equipment
Acidic water is chemically aggressive. It actively dissolves materials it comes into contact with, and pool surfaces bear the brunt. Plaster and cement-based finishes are particularly vulnerable. The acid leaches calcium out of the cement paste, creating rough, etched patches on the pool floor and walls. Over time, continued leaching can completely dissolve the cement in affected areas, turning a smooth surface into something that feels like sandpaper underfoot.
Metal components fare even worse. Heater elements, pump impellers, light fixtures, ladder rails, and any exposed copper or iron piping will corrode faster in acidic water. You may notice greenish-blue staining on pool walls or floors, a telltale sign that copper fittings are dissolving into the water. Vinyl liners won’t etch like plaster, but they can become brittle and fade faster when the pH stays low for extended periods. These repairs are expensive, and the damage is cumulative. A pool running at 6.6 for a few weeks can cause more lasting harm than most owners realize.
Why Pool pH Drops
Several things push pH downward, and they often stack up at the same time.
- Rain. Rainwater is naturally acidic, typically around pH 5.5. A heavy storm dumping into an uncovered pool can drag the pH down noticeably, especially in smaller pools.
- Heavy swimmer load. Sweat, body oils, sunscreen, and urine are all slightly acidic. A busy pool party or a week of daily use by multiple swimmers will lower pH over time.
- Chlorine tablets. Trichlor tablets, the most common type used in floating dispensers and automatic feeders, are highly acidic with a pH around 2.8. Relying on them as your sole sanitizer without balancing will steadily pull pH down.
- Low total alkalinity. This is the one most people overlook. Alkalinity acts as a buffer, resisting changes in pH. When alkalinity is low (below 80 ppm), the water has almost no ability to hold its pH steady. Even minor additions of acid, whether from rain, swimmers, or chemicals, cause the pH to swing sharply downward.
The Role of Total Alkalinity
Think of total alkalinity as your pool’s shock absorber for pH. When alkalinity is in the proper range (80 to 120 ppm), the water resists pH changes. Small additions of acid or base barely move the needle. But when alkalinity drops too low, the pH becomes unstable and can swing dramatically from one day to the next. If you find yourself constantly chasing a low pH reading, there’s a good chance your real problem is low alkalinity. Fixing the alkalinity first often stabilizes the pH on its own.
This distinction matters for how you treat the problem. Raising pH without raising alkalinity is a temporary fix. The pH will just crash again after the next rainstorm or pool party.
How to Raise Low pH
The chemical you need depends on whether your alkalinity is also low or just your pH.
If both pH and alkalinity are low, use sodium carbonate (sold as soda ash or pH increaser). For a 10,000-gallon pool, about 14 ounces of soda ash raises alkalinity by roughly 10 ppm. For a 30 ppm increase, you’d need about 2.6 pounds. Don’t try to raise alkalinity more than 50 ppm in a single treatment. Add the soda ash, let the water circulate for a few hours, then retest before adding more.
If your alkalinity is fine but your pH is low, soda ash still works, but use smaller amounts since you don’t want to overshoot the alkalinity. Add it gradually, retest after circulation, and adjust.
If your pH is fine but alkalinity is low, use sodium bicarbonate (baking soda) instead. It raises alkalinity without significantly affecting pH. These two chemicals look similar and are easy to confuse, but they do different jobs. Soda ash is not the same as baking soda.
A few practical tips: always dissolve the chemical in a bucket of pool water before adding it, broadcast it across the surface rather than dumping it in one spot, and run the pump while treating. After adding soda ash or baking soda, you can safely swim again in 20 to 30 minutes, though retesting first is a good idea.
How Often to Test
Test your pH and alkalinity at least twice a week during swimming season, and after any heavy rain or a day with lots of swimmers. Cheap test strips work for routine checks, but a liquid drop kit (DPD or phenol red type) gives more accurate readings. If you’re dealing with a persistent low-pH problem, testing daily until you stabilize the water is worth the extra minute.
Keep in mind that pH naturally drifts over time. It’s not a set-it-and-forget-it number. Consistent testing and small, regular adjustments are far easier on your pool (and your wallet) than letting the water go acidic for weeks and dealing with etched plaster or corroded equipment afterward.