August 19, 2026

pH is low but alkalinity is high - which do I fix first?

Your test strip says pH 7.0 and total alkalinity 150 ppm. The targets are 7.2 to 7.8 and 80 to 120 ppm, so one number needs to go up and the other needs to come down. Every guide you have read says "fix alkalinity first," so you look at the alkalinity chemical: baking soda. Which raises alkalinity. Already too high. Fine, fix pH instead: soda ash. Which also raises alkalinity. Both bottles in the shed push the number that is already wrong, and the internet's favorite rule seems to have no move for you at all.

It does. It just is not a bottle.

The short answer

With this combination you fix both at once, using two tools that split the job cleanly: muriatic acid works the alkalinity down, and air brings the pH back up. Acid is the only thing that lowers total alkalinity, and it drags pH down as a side effect. Aeration - splashing, jets, a return pointed at the surface - is the only common tool that raises pH without raising alkalinity. Alternate them and the two numbers walk toward their bands independently, which no chemical on the shelf can do alone.

The classic "alkalinity first" rule is not wrong here; it is just usually explained badly. Alkalinity is the lever and pH is the needle. What the rule really means is: never chase the pH number with soda ash while alkalinity sits high, because you are winding the spring tighter with every scoop.

Why both bottles push the wrong number

The confusion between the two white powders wrecks more pools than either powder does. They are different chemicals with different jobs:

The numbers make the difference concrete. In 10,000 gallons, about 1.4 lb of baking soda raises alkalinity 10 ppm while nudging pH up maybe a tenth - it is an alkalinity chemical that grazes pH. About 6 oz of soda ash raises pH around 0.2 while lifting alkalinity roughly 5 ppm - a pH chemical that grazes alkalinity. Neither is "pH up" or "alkalinity up" in pure form; they are two blends of the same two effects, in opposite proportions.

That is why the labels betray people. An owner with low pH grabs baking soda because the box says "alkalinity increaser" and alkaline sounds like the opposite of acidic. Twenty pounds later, pH has crept from 7.0 to 7.1 and alkalinity has blown past 170. And in your situation, where alkalinity is already high, both powders make the real problem worse.

The trick nobody explains: air is a pH chemical

Here is the piece that makes the whole thing click. Pool water holds dissolved carbon dioxide, and CO2 in water is carbonic acid. Your pH at any moment is largely a tug-of-war between that dissolved CO2 (pulling pH down) and the bicarbonate that alkalinity measures (holding it up). Total alkalinity is the size of the bicarbonate reservoir; pH is where the tug-of-war currently stands.

Aeration tips the war without touching the reservoir. Splash the water - jets on, a return aimed at the surface, a waterfall, even vigorous brushing - and CO2 gasses off into the air faster than it dissolves back in. Less carbonic acid means pH rises. The bicarbonate never left the water, so alkalinity reads exactly the same. Aeration is a pH-up dose that costs nothing and adds nothing.

Acid is the mirror image. Muriatic acid converts bicarbonate into CO2 and water: alkalinity drops (the reservoir shrinks), and pH drops too because that freshly made CO2 is now dissolved carbonic acid. But CO2 is temporary - aerate and it leaves. The alkalinity reduction is permanent. That asymmetry is the entire method: acid takes both numbers down, aeration brings only pH back. Run that cycle and the net movement is alkalinity down, pH held in band.

The sequence, start to finish

1. Today: aerate, add nothing. Your pH is below 7.2, and acid would push it further into the corrosive range where plaster etches and metal parts pit. Point a return at the surface and run the pump. Depending on how hard the water is churning, pH climbs over hours to a day or two. At any alkalinity of about 100 ppm or higher, aeration - not soda ash - is the correct response to low pH, full stop.

2. When pH reaches the upper half of its band (about 7.6 to 7.8): dose acid. In 10,000 gallons, roughly 26 fl oz of 31.45% muriatic acid takes alkalinity down about 10 ppm and pulls pH down a few tenths with it. Scale to your volume, and never add more than one quart per 10,000 gallons in a single addition. Dilute it first - a bucket of water, then the acid, about ten parts water to one of acid - and pour it around the deep end with the pump running.

3. Wait and retest, about 6 hours with the pump circulating. Both numbers should be lower.

4. Aerate again. pH drifts back up; alkalinity stays where the acid left it.

5. Repeat rounds 2 through 4 until alkalinity sits inside 80 to 120 ppm. From 150, that is three or four acid rounds spread over one to three weeks. There is no faster route, because there is no "alkalinity down" product - anything sold as one is just dry acid in a different costume, bound by the same chemistry and the same need for staged doses.

Two safety rules while you are in this loop: one chemical at a time, always, and keep acid hours away from any chlorine addition - concentrated sanitizer and acid meeting in strong form can produce gas you must never breathe.

How you got here in the first place

Knowing the route in tells you whether you will be back:

• Baking soda chasing. The most common cause by far - months of treating low pH readings with "alkalinity increaser," as above.
• Trichlor tabs plus hard fill water. Trichlor is strongly acidic and drags pH down continuously; if your tap water is high in bicarbonate, every top-off refills the alkalinity the tabs would otherwise erode. The result is exactly your reading: pH pinned low, alkalinity pinned high.
• A recent oversized acid dose. Acid hits pH immediately, but alkalinity is the bigger reservoir. Someone correcting high alkalinity with one heroic dose lands on low pH with alkalinity still elevated - the overshoot loop in a single afternoon. This is what the one-quart cap and the retest-before-repeating rule exist to prevent.

When the seesaw will not stop

Get both numbers in band and a high-alkalinity pool has one more trick left: it pushes pH upward on its own, because a big bicarbonate reservoir keeps off-gassing CO2 for weeks. If you find yourself adding acid every few days and watching pH climb right back, that is not this problem anymore - it is a chronic driver, and it has its own diagnosis path in why pool pH keeps rising. High alkalinity is one of the usual suspects there; so are fresh plaster and salt cells.

The reason this fix goes wrong for people is rarely the chemistry - it is sequencing and dose size, done from memory, three days apart. That is the part Basinly takes over: it scores every parameter against its band and puts the steps in order, so a low-pH, high-alkalinity test produces "aerate, nothing to add today" instead of a soda ash run; when the time for acid comes, the dose is sized to your pool's actual volume, capped at the most one round should do, and followed by a retest reminder so the next round starts from numbers instead of guesses.