What PoolBoss Says
Aerate the pool. Agitating the surface drives off carbon dioxide, which raises pH and leaves total alkalinity exactly where it is. Soda ash raises both numbers at once, so it is the right product only when alkalinity is low too. Let the alkalinity reading, not the pH reading, pick the lever.
A pool reading pH 7.1 gets one reflex from most techs: reach for the soda ash. It works, and on the wrong pool it buys you an acid visit three weeks later, because soda ash never moves pH on its own - it drags total alkalinity up with it. The reading that decides the correct lever is the alkalinity number sitting next to the pH, not the pH itself. What follows is the two-lever decision, how to aerate inside a 30-minute stop, how much soda ash a real dose takes, and why the same handful of pools keep sliding down every week.
At a glance
Key takeaways
- Let the alkalinity reading pick the lever: soda ash when both numbers are low, aeration when only pH is low.
- Aeration is the only correction that raises pH without touching total alkalinity, because it removes carbon dioxide instead of adding a chemical.
- About 6 oz of soda ash per 10,000 gallons raises pH roughly 0.2 and adds 4-5 ppm alkalinity - trivial once, close to 20 ppm a month on a weekly pool.
- Aim a return eyeball up and run any spillover or waterfall; the pool keeps aerating for the six days after you leave.
- Trichlor tabs sit near pH 2.8, so a pool on a full feeder is taking a continuous acid dose - open the feeder before you keep dosing soda ash.
- Below about 7.2 the water turns aggressive and starts etching plaster and corroding heater copper, so hold 7.4-7.6.
- Retest at the next visit rather than the same afternoon, and put the return back down once pH holds on its own.
How do I raise pH in a pool without raising alkalinity?
Aerate the water. Low pH almost always means the pool is holding more carbon dioxide than the air above it, and agitating the surface drives that carbon dioxide off. Losing it raises pH while total alkalinity stays exactly where you left it, which is the one thing nothing in the truck can do. Ideal pH is 7.4-7.6 and alkalinity belongs at 80-120 ppm on a plaster pool; when only the first of those two numbers is out of range, aeration is the whole correction.
Run it in this order on the pool, and expect the recovery to happen between visits rather than while you stand there.
- Read both numbers before you decide anything. pH 7.1 with alkalinity at 70 ppm is a different job from pH 7.1 with alkalinity at 130 ppm - the first wants soda ash and the second wants air. The alkalinity reading is the one choosing your product.
- Aim a return eyeball up so the jet breaks the surface. That single adjustment turns the pool's own pump into an aerator that keeps working for the six days after you drive away, which is where most of the pH recovery actually happens.
- Switch on anything that moves water through air - a spa spillover, a waterfall, deck jets, a fountain head. A spillover running a few hours a day moves pH further than anything you can pour during the 25 minutes you are on site.
- Brush the walls and floor while you are there. Brushing agitates far more surface than techs give it credit for, and on a pool with no water feature it is the only aeration you can add by hand.
- Retest at the next visit and put the return back down once pH holds at 7.4. Aeration stalls out on its own as the water reaches equilibrium, so it will not overshoot the way a dose does - but a return left aimed up for a month keeps pushing pH toward 8.0.
Soda ash raises both numbers; aeration raises only pH
That asymmetry is the entire decision. Soda ash, sodium carbonate, sold on the shelf as pH increaser, dissolves into carbonate - which pushes pH up and adds total alkalinity in the same motion. Aeration works by removing carbon dioxide rather than adding anything, so pH climbs and alkalinity does not move at all. One lever raises both numbers, the other raises one. Which lever is correct is decided by the number you were not trying to fix.
It also means this correction has to be judged on the pair of readings rather than the one that looked wrong. A pool that needs pH raised twice a month is telling you something about its tab feeder, its fill water, or its alkalinity, and none of that is visible in a single visit. If you log pH and total alkalinity together at every visit, the pattern across a month is what picks the lever - and it is also the difference between correcting a pool and maintaining one.
| What you're reading | Correct lever | What happens to the other number |
|---|---|---|
| pH low, alkalinity low (under 80 ppm) | Soda ash | Alkalinity rises too, which is what you want here |
| pH low, alkalinity normal (80-120 ppm) | Aeration | Alkalinity does not move |
| pH low, alkalinity high (over 120 ppm) | Aeration only | Soda ash adds alkalinity you will pay acid to remove |
| pH high, alkalinity high | Acid, then aerate | Both fall; aeration brings pH back up |
| pH fine, alkalinity low | Sodium bicarbonate | pH barely moves |
Soda ash is the right product only when alkalinity is low too
Reach for it when both numbers are down and leave it on the truck when they are not. As a working figure, about 6 oz of soda ash per 10,000 gallons raises pH by roughly 0.2, and that same dose adds about 4-5 ppm of total alkalinity. On one visit the alkalinity bump is nothing. On a pool that needs soda ash every week it is close to 20 ppm a month, and that is how a pool sitting at 110 ppm quietly becomes a pool at 170 ppm that you are now buying acid to bring back down.
Those numbers are typical rather than exact, and they are only as good as the gallonage you plug into them, so size the dose to the pool's real volume rather than the figure somebody wrote on a route sheet years ago. Dose to about two thirds of what the math says and retest at the next visit. Soda ash is slow to finish moving and easy to run past 7.8, where chlorine starts losing its bite and the tile line starts collecting scale.
The pool that needs the opposite correction is a different job with the same mechanism running backwards - bringing alkalinity down without crashing pH is acid plus aeration, where this one is aeration alone. Same two levers, opposite starting point.
Take a two-tech operation running 165 pools across Henderson and Green Valley, Nevada. A Henderson pool comes in at pH 7.1 with total alkalinity at 130 ppm. Soda ash would have the pH fixed by the time the tech backs out of the driveway, and it would push a pool that is already high on alkalinity higher - every week, on a route where the fill water is doing the same thing from the other side. The right call costs nothing: aim the return up, switch on the spillover, brush, and let the pool climb on its own between now and Thursday. Same tech, same 25 minutes, and the pool does not come back as an acid job in three weeks.
| Pool volume | Soda ash | Alkalinity it also adds | Pour two thirds of this |
|---|---|---|---|
| 10,000 gal | about 6 oz | 4-5 ppm | about 4 oz |
| 15,000 gal | about 9 oz | 4-5 ppm | about 6 oz |
| 20,000 gal | about 12 oz | 4-5 ppm | about 8 oz |
| 25,000 gal | about 15 oz | 4-5 ppm | about 10 oz |
The same pools slide down every week, and rain is not why
Rain gets the blame and rarely deserves it on a Sunbelt route. Three things actually pull pH down visit after visit, and all three are on your side of the fence. Trichlor tabs are the biggest: stabilized chlorine tablets sit around pH 2.8, so a pool on a full feeder is taking a continuous small acid dose, and the pool with three tabs in the skimmer is the pool you keep correcting. Second is over-correction - a tech who answers a high pH with a full acid dose every week walks alkalinity down alongside it until there is nothing left to buffer with. Third is low alkalinity itself: under about 60 ppm, pH has no resistance and swings on almost nothing, and that is the one case where the honest answer really is soda ash rather than air.
The tab feeder is worth opening before you dose anything, because trichlor does two things at once - it acidifies the water and it adds cyanuric acid that never leaves on its own. The pool whose pH you keep raising is usually the same pool climbing toward a stabilizer problem you will have to drain for. Per-pool history is what makes that visible: a pH column reading 7.1, 7.2, 7.0, 7.1 across four weeks is a diagnosis, where any single one of those visits just looks like a reading.
Low pH costs more than a bad reading
A pool sitting at 7.0 is slowly dissolving itself, and the bill lands on the customer's plaster and the customer's heater. Below roughly 7.2 the water turns aggressive: it etches plaster, pulls grout out of the tile line, wrinkles vinyl liners, and corrodes the copper inside a heat exchanger. A heater replacement runs $2,500-$4,500 and a plaster resurface $5,000-$10,000 on a typical residential pool, and both of those conversations tend to start with a service company that let pH ride low for a season.
The pH number by itself is not what does the damage - it is what low pH does to the water's saturation balance once calcium and alkalinity are in the picture. A pool at 7.2 with 300 ppm calcium hardness and 110 ppm alkalinity is fine. A pool at 7.2 with 150 ppm calcium and 60 ppm alkalinity is pulling calcium out of the plaster to make up the shortfall. The low-pH pools genuinely worth worrying about are the soft-water ones.
Etching is also the failure mode you can be blamed for, because it accumulates quietly and then becomes visible all at once. A season of readings showing the pool held 7.4-7.6 is the difference between a warranty conversation with the builder and an argument with your customer about whose chemistry did it.
FAQ
Frequently asked questions
Can I use baking soda to raise pH instead?
Baking soda is the wrong tool for pH, though it is the right one for alkalinity. Sodium bicarbonate raises total alkalinity by about 10 ppm per 1.4 lb in 10,000 gallons while moving pH only slightly, usually a tenth or two, because it settles the water toward roughly 8.3 rather than driving it there. Soda ash, sodium carbonate, is the pH product: a fraction of the weight moves pH several times further. In practice the two get used for different readings. If alkalinity is low and pH is acceptable, use bicarbonate and let pH sort itself out. If pH is low and alkalinity is fine, neither product is right and aeration is the answer. If both are low, soda ash handles both at once, which is the only situation where it is genuinely the efficient choice.
How long does aeration take to move pH on a service pool?
Hours to days, not minutes, which is why it is set up rather than supervised. With a return aimed up and a spillover running, a typical pool climbs something like 0.2-0.4 in the first 24-48 hours and keeps going more slowly after that, so a pool left at 7.1 on Tuesday is often near 7.5 by the weekend. The speed depends on how much excess carbon dioxide the water is carrying: a pool with higher alkalinity holds more of it and aerates faster, while a pool at 60 ppm alkalinity has less to give off and moves sluggishly. That is worth knowing before you decide aeration failed. If a week of aggressive aeration barely moves pH, the pool is probably short on alkalinity, and bicarbonate is the missing step.
Should I fix pH or total alkalinity first?
Alkalinity first in almost every case, because alkalinity is what holds pH in place. Correcting pH on a pool sitting at 50 ppm total alkalinity is work you will repeat next week, since there is nothing buffering the number you just moved. Bring alkalinity into the 80-120 ppm range with sodium bicarbonate, then read pH again - it will often have drifted into range on its own, and whatever is left is a much smaller correction. The exception is a pool that is actively aggressive, below about 7.0, where the water is etching plaster right now. There, get pH up immediately with soda ash, which raises alkalinity at the same time and moves you toward both targets, then fine-tune alkalinity at the next visit once the surface is out of danger.
Does a salt system make pH drift up on its own?
Yes, and consistently enough that a salt pool reading low pH is worth a second look. A salt cell generates chlorine through electrolysis, and the process releases hydrogen gas at the cell while the return flow stays turbulent, which aerates the water continuously. The result is the same mechanism described above running full time, so salt pools typically climb toward 7.8-8.0 and need acid rather than soda ash. If a salt pool is reading 7.1, something else is overriding that drift: tabs added on top of the cell, a heavy rain dilution, very low alkalinity, or a cell that has stopped producing. Chase the cause rather than dosing the number, because a salt pool that needs pH raised every week has a problem the soda ash is only masking.
Are borates worth using to hold pH steady on a problem pool?
On the two or three pools you keep correcting, usually yes. Borates held at 30-50 ppm act as a secondary buffer that resists pH movement in both directions, so a pool that used to need attention every visit needs it every month or two instead. It is a one-time cost of roughly $100-150 in product on a 15,000-gallon pool plus a borate test strip, and it comes with a mild algaestatic side benefit. Two cautions. Borates are not a substitute for correct alkalinity - a pool at 50 ppm alkalinity still needs bicarbonate first, and borates layered on top of bad buffering just make the problem harder to read. And borates are toxic to dogs if a pet drinks pool water regularly, so it is worth asking the customer before you add them.
Does the type of shock I use change pH?
Every chlorine product shifts pH, and on a pool that keeps sliding down the shock you choose can do part of the correction for free. Cal-hypo, calcium hypochlorite, is basic and pushes pH up while adding calcium hardness, which suits a soft-water pool that runs low. Dichlor is acidic and pulls pH down while adding cyanuric acid, so it is the wrong shock for a pool you are already fighting to raise. Liquid chlorine is close to pH-neutral in net effect once the chlorine is consumed, despite going in strongly basic. Switching a chronically low-pH plaster pool from dichlor to cal-hypo often removes the need to dose pH at all, which is one product change rather than a standing weekly correction.
