How to Raise Pool pH: Soda Ash, Bicarbonate and Aeration

A pool that reads low on pH is telling you something more specific than "add something alkaline". Below about 7.2 the water is actively aggressive: it pulls calcium out of plaster and grout, attacks the soft metals in a heater's heat exchanger, corrodes ladder anchors and light rings, stings eyes, and quietly eats through equipment that costs far more to replace than the chemical would have cost to prevent. Unlike high pH, which mostly degrades performance, low pH does damage that does not undo itself when the reading comes back.

The complication is that pH is rarely the real problem. It is the number that moves first and most visibly, but what usually decides whether it stays put is total alkalinity, which nobody looks at until the pH has bounced three weeks running. Raising pH in a pool with low alkalinity is like straightening a picture on a wall that is settling. It works for a day, and then it does not, and you will keep doing it until you fix the thing underneath.

What low pH is doing while you are not looking

The visible symptoms arrive late. Swimmers complain about stinging eyes and dry skin, and chlorine seems unusually harsh even at ordinary levels, because at low pH a larger share of the free chlorine is in its most aggressive form. Those are the complaints that get a pool looked at. The damage that matters started earlier and is cumulative.

On a plastered pool, aggressive water dissolves calcium carbonate out of the surface. The finish goes rough underfoot, then mottled, then in bad cases exposes aggregate. Grout between tiles washes out along the waterline. On the equipment pad the story is worse and faster, because a gas heater's heat exchanger is usually copper, and copper in acidic water gives up metal steadily. A heater run for one season on water at 6.9 can fail in a way that looks like a manufacturing fault and is not. Vinyl-liner pools escape the etching but not the metalwork: ladder bolts, light rings, and the screws holding the skimmer faceplate all corrode, and the staining that follows is copper leaching out of the pipework and depositing on the surface.

pH and total alkalinity

pH measures how acidic or basic the water is right now, on a scale where 7.0 is neutral. Total alkalinity measures the water's capacity to resist a change in pH, mostly as dissolved bicarbonate. Alkalinity is the buffer; pH is the reading. A pool with healthy alkalinity moves pH slowly and predictably. A pool with low alkalinity swings on very little, which is why the same pool can read 7.0 one week and 8.0 the next without anyone doing anything unusual.

The reading that decides the fix: total alkalinity

Before anything goes in the water, you need two numbers rather than one. pH on its own does not tell you which product to reach for, because the same low reading calls for a different answer depending on what alkalinity is doing underneath it. Testing both in the same sample takes an extra thirty seconds and is the difference between a correction that holds and one you will repeat next week.

The target ranges below are the residential defaults used across the trade, and they are the same defaults Pool Runs classifies a logged reading against, marking it low, in range or high. They are a starting point rather than a rule: a heavily used commercial pool, a salt pool, and a shaded vinyl pool in a cold climate all behave differently, and an operator who has run the same pool for three seasons usually has a better target for it than any table does.

With both numbers in hand the decision resolves cleanly. Low pH with alkalinity also below 80 means alkalinity is the job and pH will follow it up on its own. Low pH with alkalinity comfortably inside range means pH is genuinely the only thing out, and it can be moved directly. Low pH with alkalinity above range is the awkward case, and it is nearly always a pool being dosed with acid too often, where the acid has pulled pH down faster than alkalinity: leave it alone, aerate, and let pH climb back without adding more buffer to a pool that already has too much.

Soda ash or sodium bicarbonate: they are not interchangeable

Both products are sold in white tubs, both are a powder, and on a shelf they look like the same thing at two prices. In the water they do quite different jobs, and reaching for the wrong one is the most common way a pH correction goes sideways. Sodium carbonate, sold as soda ash or pH increaser, moves pH hard and alkalinity only modestly. Sodium bicarbonate, sold as alkalinity increaser and chemically identical to baking soda, moves alkalinity hard and pH only slightly.

That asymmetry is the useful part. A pool sitting at 7.0 with alkalinity at 60 needs its buffer rebuilt, and bicarbonate does that while nudging pH up as a side effect, often far enough that no second product is needed at all. A pool at 7.0 with alkalinity at 100 has a perfectly good buffer and simply needs pH moved, which is what soda ash is for. Using soda ash on the first pool overshoots pH while leaving the underlying instability exactly where it was, so the reading is fine on the day and wrong again a week later.

How it goes in matters nearly as much as which one goes in. Soda ash thrown dry across the surface raises pH violently in a small volume of water before it disperses, and in a pool with any calcium at all that produces a cloud that takes days to clear and can leave scale on the surface where the powder settled. Pre-dissolving in a bucket of pool water and pouring it slowly in front of a running return solves that completely, costs two minutes, and is the single habit that separates technicians whose pools stay clear from ones who are forever explaining cloudiness.

Aeration: raising pH without adding anything

Pool water holds dissolved carbon dioxide, and carbon dioxide in water is mildly acidic. Agitating the surface lets it escape into the air, and as it leaves, pH rises. Total alkalinity does not move, because no buffer has been added or consumed. That is an unusual and useful property: it is the only common way to raise pH in isolation, and it costs nothing but time.

This makes aeration the correct answer to the case that stumps people, which is a pool with low pH and alkalinity already at or above the top of its range. Adding soda ash there fixes pH and pushes alkalinity further out. Adding nothing and aerating fixes pH and leaves alkalinity alone. The same mechanism explains why water features, aggressive returns and heavy swimmer use all tend to drive pH upward over a season, and why a pool with a fountain running most of the day rarely reads low.

1. 1

   Angle the returns upward

   Turn the eyeball fittings so the flow breaks the surface rather than pushing along underneath it. Surface agitation is the entire mechanism, so a jet that churns is worth several that merely circulate.

2. 2

   Run anything that moves air through water

   Spa jets, waterfalls, deck jets, bubblers and fountains all accelerate it. On a pool with none of these, a submersible pump or even a garden hose aimed across the surface from the deck will do the job more slowly.

3. 3

   Give it a day or two, not an hour

   Aeration is gradual. A pool with good circulation can climb two or three tenths of a point over a day of running; one with only standard returns may take a weekend. This is a between-visits job rather than a stop-and-wait one.

4. 4

   Retest before you stop

   Aeration does not self-limit. A pool left aerating for a week can drift above 8.0, and then you are adding acid to fix the fix. Check the reading and set the returns back down once it is inside range.

When pH keeps falling: find the cause, not the dose

A single low reading is a correction. The same pool reading low every visit is a question, and answering it is worth more than any amount of accurate dosing, because a pool with a persistent acid source will consume product indefinitely and still spend most of its life out of range. There are only a handful of realistic causes, and they are usually distinguishable from the pattern rather than from the pool.

Acid overdosing is the most common, and the most easily missed, because it is self-inflicted and looks like diligence. A technician who adds muriatic acid whenever pH reads above 7.6 will, over a season, drive alkalinity steadily down until the pool has no buffer left and swings on nothing. The tell is alkalinity falling month over month while acid usage stays constant. Heavy rain does something similar on an outdoor pool, since rainwater is mildly acidic and arrives in volume, and a pool topped up frequently from a fill source that is itself soft and low-alkalinity will drift down no matter what is added at the pool.

Trichlor tablets are an acid source people forget, because they are thought of as chlorine rather than as chemistry. Trichlor is strongly acidic, so a pool running tablets continuously in a feeder is being dosed with acid continuously as well, and on a small pool with a full feeder that is enough to hold pH permanently low while also driving stabiliser up. A salt pool has the opposite reputation and a more complicated reality: salt cells push pH up in normal operation, so a salt pool that reads persistently low is unusual and usually points at the fill water or at rain rather than at the cell.

What this looks like across a route

On one pool this is a decision made standing on the deck with two test results in front of you. Across forty pools serviced weekly by two or three people it is a records problem, because the thing that identifies an acid source is a trend across visits, and a trend is invisible to whoever happens to be at the pool today. The technician who sees alkalinity at 70 has no way of knowing whether that is a pool recovering from 50 or sliding from 95, and those two pools need opposite actions.

The practical fix is unglamorous: capture the readings and the dose at the stop, against the property rather than in a notebook, and give each pool its own targets rather than running the whole book off one table. A shaded vinyl pool on soft fill water and a sun-exposed plaster pool on a tablet feeder are not the same pool and should not have the same trigger points. Once the history exists, the pools with a genuine acid source separate themselves out from the pools that just needed a correction, and only the first group needs anyone to think hard about it.

The mirror image of this problem is far more common on most routes, and worth reading alongside it: pH that climbs relentlessly has its own set of causes and its own correct answers, covered in how to lower pool pH. Both start from the same place, which is a sample taken properly and read honestly, and how you test pool water decides more about the outcome than which tub you carry.

Two neighbouring topics change the answer enough to be worth knowing before you dose. On a salt pool the generator pushes pH the other way as a matter of course, so the whole diagnosis inverts and salt water pool maintenance sets out what that pool actually needs. And because pH decides how hard your chlorine works, a pool corrected on pH often needs less of it than the same pool did last week, which is the arithmetic in how much chlorine to add to a pool.

Pool Runs keeps every logged reading and the dose that followed it on the property record, and classifies each reading against the default ranges above, so the alkalinity trend that identifies an acid source is visible without anyone assembling it by hand. The targets themselves are not configurable per pool yet, so the trigger points you set for an individual pool stay your own call. If your readings currently live on a clipboard, the mobile app technicians use at the stop is where that part of the job belongs.