Total Alkalinity and pH: Which Should You Fix First?

pH is the reading people chase. Total alkalinity is the reading that decides whether they will ever catch it. Correcting them in the wrong order means doing the same job again next week, and the week after that.

What each one measures

pH is how acidic or basic the water is, on a scale where 7 is neutral. Pool water should sit between 7.2 and 7.8, ideally 7.4 to 7.6. It is a single instantaneous reading and it can move within hours — after a rain shower, after a chemical dose, after a busy afternoon.

Total alkalinity measures something different: the water’s capacity to resist a change in pH. It is not “how basic the water is”, which is the misunderstanding that sends people in the wrong direction. It is how hard the water will fight you when you try to move pH.

A useful way to hold the two apart: pH is the position of a door, alkalinity is the stiffness of its hinge. A door with a loose hinge swings on any draught. A door with a seized hinge will not move however hard you push. Neither is useful, and in both cases adjusting the door position is not the fix.

Why the order matters so much

With alkalinity in range, pH drifts slowly and predictably. You correct it now and then and it stays where you put it. This is what a well-behaved pool feels like, and it is almost entirely a function of alkalinity being right.

With alkalinity too low — below about 60 ppm — pH becomes unstable. A rain shower swings it. A single chemical dose swings it. A busy weekend swings it. This is pH bounce, and no amount of pH adjustment fixes it, because pH is the symptom and alkalinity is the cause. Owners in this situation often describe adding pH increaser and pH decreaser in the same week, which is exactly what low alkalinity produces.

With alkalinity too high — above about 140 ppm — pH climbs relentlessly and will not stay down however much acid goes in. You are not fighting pH at that point; you are fighting the buffer itself, and the buffer is much larger than the thing you are adjusting.

The ranges worth holding

The two exceptions matter more than they look. If you run a salt pool at 120 ppm alkalinity you will be adding acid constantly, because the generator raises pH as an inherent by-product and a large buffer resists your correction while doing nothing to resist the rise. Dropping alkalinity to 70 or 80 is the standard answer, and it usually halves the acid demand.

How pH decides whether your chlorine works

This is the part that turns pH from a number on a strip into something worth caring about.

Chlorine exists in pool water in two forms. Hypochlorous acid is the one that sanitises effectively. Hypochlorite ion is far weaker — by most estimates something like eighty times less effective. The split between them is set by pH, and by nothing else.

Your test kit reading has not changed across that table. The pool still reads 3 ppm free chlorine at pH 8.0 just as it did at 7.2 — but three-quarters of it is now doing very little. This is precisely how a pool with “plenty of chlorine” still goes green, and it is the mechanism behind almost every case of high pH.

It also explains why fixing pH is often the cheapest thing you can do. Bringing pH from 8.0 down to 7.4 roughly doubles the effective strength of chlorine you have already paid for.

Raising alkalinity

Sodium bicarbonate — baking soda, sold as alkalinity increaser — raises alkalinity with only a small upward effect on pH. It is forgiving, inexpensive, and genuinely hard to get badly wrong.

Raising alkalinity by 10 ppm in 10,000 gallons takes roughly 1.5 lb of sodium bicarbonate. Add it with the pump running, broadcast across the deep end rather than poured into one spot, and re-test after a full turnover rather than an hour later. The alkalinity calculator works out the dose for your volume and your current reading.

If alkalinity is very low — under 50 ppm — raise it in stages across several days rather than in one dose. Large single additions can cloud the water temporarily, and they make it harder to see what effect you are actually having.

One practical warning: soda ash (sodium carbonate, sold as pH increaser) is not the same thing. It raises pH sharply and alkalinity only slightly. Reaching for it when you meant to raise alkalinity is a common and frustrating mistake, because you end up with high pH and the original low-alkalinity problem intact.

Lowering alkalinity — the part that confuses everyone

There is no chemical that lowers alkalinity without also lowering pH. Muriatic acid and dry acid both reduce the two together. So the obvious question: how do you lower alkalinity while keeping pH where you want it?

The answer relies on a quirk of the chemistry. Aeration raises pH without changing alkalinity. Acid lowers both. Alternate the two and you walk alkalinity down a step at a time while pH returns to where it started after each step.

1. Add acid to bring pH down to about 7.0–7.2. Alkalinity falls with it. Add the acid slowly, in a thin stream over the deep end with the pump running — not dumped in one place, which can etch plaster directly beneath it.
2. Aerate the water. Run a fountain, aim the return jets upward so they break the surface, switch on a waterfall or the spa jets. pH rises back toward 7.4 over several hours; alkalinity stays where the acid left it.
3. Test. If alkalinity is still above range, repeat from step one.

It takes patience. On a badly out-of-range pool it can take several days and three or four cycles. But it is the only method that works, and it explains why so many owners add acid week after week, watch pH come down and bounce back, and conclude that alkalinity is simply stuck.

The reason aeration works is that high alkalinity water holds excess dissolved carbon dioxide. Agitating the surface lets that CO2 escape, which raises pH — the same reason a fizzy drink goes flat and tastes less sharp. Nothing is added or removed except gas, so alkalinity is untouched.

What pushes each one out of range

Two of those rows explain most real-world complaints. A pool with a waterfall or a heavily aerated feature will have pH climbing constantly, and there is nothing wrong with it — that is physics, not a fault. A pool fed by trichlor tablets will have both readings drifting down, which is why tablet-fed pools so often need alkalinity increaser as routine.

Your fill water is worth testing once, and almost nobody does. Municipal supplies vary enormously in alkalinity and hardness. If yours arrives at 200 ppm alkalinity you will be fighting it every time you top up, and that changes what a realistic target is. The EPA publishes the national drinking water regulations, and your own utility publishes an annual water quality report with the actual figures for your area — usually free, usually on their website, and worth ten minutes.

Reading your test kit properly

Alkalinity is one of the readings test strips handle worst. The colour blocks sit close together and the difference between 80 and 120 ppm can be a shade most people cannot call reliably in daylight, let alone at dusk.

A drop-based titration kit is far more trustworthy here. You count drops until a colour change, and the count is a number rather than a judgement. For a reading that determines how your whole pool behaves, that difference is worth the modest extra cost.

Whatever you use: test at the same time of day, from the same depth — elbow-deep, away from returns and skimmers — and replace reagents every season. Test strips in particular degrade once the bottle has been opened, and a strip that has been sitting in a damp pool shed since last summer is not measuring anything useful.

A practical weekly routine

1. Test pH and free chlorine twice a week during swimming season.
2. Test total alkalinity weekly at first; monthly once the pool is stable.
3. If both are out of range, correct alkalinity first and wait a full turnover before judging pH at all.
4. Adjust one thing at a time, and re-test before the next adjustment.
5. If you find yourself correcting pH more than once a week, stop correcting pH. The problem is alkalinity.

That last rule is the single most useful thing in this article. Repeated pH correction is not maintenance; it is a symptom, and it is telling you to look at a different number.

When the water is already damaged

Low alkalinity is not merely inconvenient. Below about 60 ppm the water turns corrosive: it attacks metal fittings, ladder anchors, light rings, pump seals and — most expensively — the copper heat exchanger inside a heater. It will also etch plaster, leaving a surface that feels rough or chalky underfoot.

None of that damage reverses. A heater that has been run in aggressive water for a season may fail years early, and by the time it fails the water chemistry that caused it is long since corrected and forgotten. If your alkalinity has been low for a while, correcting it stops further damage but does not undo what has happened — see low alkalinity and low pH for what to look for.

High alkalinity is less destructive but more persistent. It drives scaling, particularly in combination with high calcium and high pH, and scale inside a heat exchanger quietly raises your heating bill every year. See calcium hardness for how the two interact.

Salt pools, tablet pools and why they drift in opposite directions

Two very common setups produce opposite chemistry problems, and recognising which one you have explains most of what your pool does week to week.

A salt chlorine generator raises pH continuously. The electrolysis that makes chlorine also produces a by-product that pushes pH upward, and there is no way to switch that off — it is inherent to the process. A salt pool therefore needs acid regularly, and the owner who reads that “salt pools are low maintenance” is often surprised by how often.

The standard answer is to run alkalinity lower than a traditional pool: 70 to 80 ppm rather than 100 to 120. A smaller buffer means the acid you add moves pH further, so you add less of it, less often. It feels counter-intuitive to deliberately reduce a reading, but it is the correct move on this equipment.

A trichlor tablet pool does the reverse. Trichlor is acidic, so every tablet nudges both pH and alkalinity down. Left alone, a tablet-fed pool drifts toward the aggressive end, which is exactly the condition that etches plaster and corrodes heaters. These pools need alkalinity increaser as routine rather than as a correction, and they benefit from a target at the higher end of the range.

Knowing which of these you are is worth more than any individual test result, because it tells you which direction to expect the drift and therefore what to check for.

A first-week routine for a pool that keeps bouncing

If pH will not settle, this sequence usually resolves it within a week, and it works because it deliberately changes only one thing at a time.

1. Day one. Test everything, including cyanuric acid. Write the numbers down. If CYA is very high, note that your alkalinity reading is overstated and expect less buffer than the number suggests.
2. Day one, later. Correct alkalinity only. Do not touch pH, however wrong it looks.
3. Day two. Test again after a full turnover. Very often pH has moved into range by itself, and the job is finished.
4. Day three. If pH is still out, correct it now, and only now. One adjustment.
5. Day five. Test both. If they have held, the pool is stable and you can return to a normal routine.
6. Day seven. Test once more. Two stable readings four days apart is what “solved” looks like.

The discipline is in step two. Almost everybody corrects pH on day one because it is the reading that looks wrong, and that single decision is what turns a one-week fix into a season of chasing.