If your pool is new, has a waterfall, fountain or any kind of jet that stirs up the water, you've probably noticed the pH tends to drift upward on its own, week after week, even when you correct it properly. It's not that your maintenance is failing: it's a natural physical process known as CO2 degassing, and understanding it saves you from correcting the pH more often than you actually need to.
CO₂ escapes and pH rises
What CO2 degassing is
All water contains dissolved carbon dioxide (CO2), much like a fizzy drink. How much gas the water can hold depends on the balance between the CO2 dissolved in it and the CO2 present in the surrounding air, which is, in essence, what Henry's Law describes: a dissolved gas tends to escape from water into the air until both sides reach equilibrium. When pool water arrives with more dissolved CO2 than it "should" have at equilibrium with the atmosphere, that excess is released little by little into the air, invisible gas slipping out through the surface.
Why that makes pH rise
The CO2 dissolved in water forms carbonic acid, which is precisely what keeps the pH somewhat lower. As CO2 escapes into the air, that source of acidity gradually disappears and the water becomes progressively more basic: pH climbs. No new product needs to enter the water, it's simply the pool "breathing" its way toward natural equilibrium with the atmosphere.
When it's most noticeable
- Freshly-filled pools. Tap or well water usually arrives with a CO2 load well above equilibrium, so in the first weeks of a new pool it's common to have to correct pH several times in a row until it settles.
- Pools with waterfalls, fountains or decorative jets. Anything that breaks the water's surface or brings it into close contact with the air speeds up gas exchange dramatically, and with it, degassing.
- Pools heavily aerated by the filtration system itself, for example with returns that create a lot of surface turbulence.
- Heated pools or ones very exposed to wind, where constant water movement likewise favors gas exchange.
Why it isn't a maintenance failure
It's tempting to assume that if pH keeps rising, something is being done wrong, but it's actually the opposite: it's an expected consequence of how water behaves. Degassing isn't indefinite, as the water approaches equilibrium with the air, the pace of the rise slows down, until pH stabilizes and stops drifting on its own so much. It's the exact same reason the Langelier Saturation Index calculator tends to read worse on a freshly-filled pool than on one that's already settled.
When it actually is worth worrying about
Normal degassing slows down over time. If pH keeps climbing week after week without ever settling, or if the rise is very sharp in a very short time, that no longer fits a simple move toward equilibrium and it's worth checking other causes: total alkalinity that's too high (which "pushes" pH upward harder than normal), a maintenance product with an alkalinizing effect used in excess, or a constant top-up of fresh water with an already-high starting pH. In that case, the practical move is to review the full set of water parameters rather than simply lowering the pH over and over.
Frequently asked questions
Why does my pool's pH rise even when I haven't added anything?
Because the water contains dissolved CO2 that slowly escapes into the air until it reaches equilibrium, a physical process called degassing. As the water loses CO2, it becomes slightly more basic and pH rises gradually and steadily.
Why does it happen more in new pools or ones with waterfalls?
Because freshly-filled water usually arrives with more dissolved CO2 than it will have at equilibrium, and because anything that agitates or aerates the water, waterfalls, fountains, jets, heavy skimmer turbulence, speeds up the gas exchange with the air and brings that pH rise on sooner.
When should I actually worry about a rising pH?
When the rise is very fast, very large, or never settles down over time. Normal degassing slows as the water approaches equilibrium; if pH keeps climbing week after week with no let-up, it's worth checking total alkalinity and the rest of the water's parameters.