Bob Lowry: The Truth About Persistent High pH in Pools

Constant high pool pH is one of the most common pool water chemistry problems, and it usually is not “mystery water” at all. A persistent pH rise is often driven by carbon dioxide off gassing, which is controlled largely by total alkalinity (TA). When TA is high, the water is effectively overcarbonated, so CO2 wants to escape until it reaches equilibrium with the air above the pool. As CO2 leaves, the pH rises, which leads to the familiar cycle of adding acid, watching pH drop, then seeing it climb again. If you want stable pH, the strategy is less about chasing numbers and more about reducing the conditions that accelerate CO2 loss.

The practical takeaway is to rethink alkalinity targets when you are dealing with chronic high pH. Many “standard ranges” for total alkalinity are too broad to solve real-world pH drift, especially in modern pools with lots of aeration. A target around 90 ppm TA can be a starting point, but if pH still climbs, lowering TA to 80 ppm or even near 70 ppm can reduce the rate of CO2 off gassing and keep pH stable. The goal is not to hit a textbook range, but to find the TA that minimizes pH rise in that specific pool. Lowering TA is also about patience and testing: adjust, observe the pH trend, then fine-tune rather than making big swings.

Aeration and turbulence can overwhelm otherwise good chemistry, which is why feature pools often struggle the most. Negative edge pools, waterfalls, spillways, scuppers, spa jets, blowers, and rock cascades all increase gas exchange at the water surface. More turbulence means faster CO2 loss, and faster CO2 loss means faster pH rise. You can see it dramatically in a spa: lower the pH with dry acid, run jets and blowers, then watch pH rebound quickly. If a pool has extreme aeration like a large negative edge with a big drop, stabilizing pH may require equipment solutions such as CO2 injection to replace the CO2 being stripped out, or operational changes that reduce runtime and turbulence where possible.

Saltwater chlorine generators add another layer because they raise pH in two ways: they create hydroxide and they create hydrogen gas bubbles, which adds continuous aeration inside the cell. One approach is to reduce generator runtime by maintaining the right free chlorine level for the pool and using tools that lower chlorine demand. Borates are often discussed here because they can act as an algostat and also slow pH rise, effectively adding buffering that helps pH stability. The conversation also highlights cyanuric acid (CYA) management for salt pools, since adequate CYA protects chlorine from UV loss and can improve efficiency when paired with appropriate free chlorine targets. Finally, new plaster pools commonly experience pH rise during curing and plaster hydration, which can last many months. And despite long-held myths, liquid chlorine and calcium hypochlorite do not create a lasting pH increase after chlorine is consumed, because the chemistry largely cancels out over time.