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Hot Tub Water Chemistry: A Cincinnati Beginner’s Guide
October is when the hot tubs on our floor start going home. The pool is closed, the evenings are cold, and a spa is the one piece of backyard water that gets better as the weather gets worse. It is also the point where a lot of new owners find out that hot tub water chemistry is not pool chemistry shrunk down. A spa holds 300 to 450 gallons, runs at 100 degrees or more, and gets four people in it at once. Everything that happens to the water happens fast.
We have been testing water at the counter on Reading Road since 1966, and the spa samples that come in cloudy, foamy or smelling like a locker room almost always trace back to one of four numbers being off. This guide covers those numbers, the order to fix them, the bromine versus chlorine decision, and what Cincinnati tap water and outer-county well water do to a fresh fill. It is for someone who just bought a hot tub in Cincinnati and wants to keep it clear without turning water care into a second job.
The short version
- Balanced spa water means four numbers in range: total alkalinity 80 to 120 ppm, pH 7.2 to 7.8, calcium hardness 150 to 250 ppm, and sanitizer at bromine 3 to 5 ppm or free chlorine 1 to 3 ppm.
- Fix them in that order. Alkalinity steadies pH, pH decides how well the sanitizer works, and sanitizer added to unbalanced water is money poured down the drain.
- Bromine is the easier sanitizer for most hot tubs because it keeps working at high temperature and high pH. Chlorine costs less and acts faster but needs more attention.
- Cincinnati city water fills a spa at roughly 125 to 140 ppm calcium hardness, which is below the spa range, so most city-filled tubs need calcium added, not removed. Well water in the outer counties is the opposite problem.
- Water older than about four months cannot be balanced back to life. Drain it, clean the shell, and start fresh.
What are the four numbers that keep hot tub water balanced?
Hot tub water is balanced when total alkalinity, pH, calcium hardness and sanitizer are all inside their ranges at the same time. Alkalinity holds pH steady, pH controls comfort and sanitizer strength, calcium protects the shell and heater, and the sanitizer kills what four warm bodies bring into 400 gallons of hot water.
Every test strip and every bottle in the spa chemicals aisle is aimed at one of these four readings. Here they are, with the ranges we use at the counter and what each one actually does in a hot tub.
| Reading | Spa range | What it does | What goes wrong when it drifts |
|---|---|---|---|
| Total alkalinity | 80 to 120 ppm | Buffers pH so it does not swing every time someone gets in | Low: pH bounces, water gets corrosive. High: pH climbs and sticks, scale forms |
| pH | 7.2 to 7.8, ideally 7.4 to 7.6 | Sets how well the sanitizer works and how the water feels on skin and eyes | Low: stings eyes, eats heater elements and jet seals. High: sanitizer goes weak, water clouds, scale on the shell |
| Calcium hardness | 150 to 250 ppm | Keeps the water from pulling minerals out of the acrylic, plumbing and heater | Low: foaming, etched surfaces, corroded heater. High: chalky scale at the waterline and on the heater |
| Sanitizer | Bromine 3 to 5 ppm or free chlorine 1 to 3 ppm | Kills bacteria, which multiply far faster at 102 degrees than in a pool | Low: cloudy water, odor, rash risk. High: bleached suits, irritated skin, cover damage |
The thing to understand about a spa is scale. A 20,000 gallon pool shrugs off two swimmers. A 400 gallon spa with four people in it carries fifty times the bather load per gallon, at a temperature that makes bacteria happy and sanitizer disappear. The four numbers a pool owner checks once a week need checking two or three times a week in a hot tub. If you own both, our post on how often to test pool water covers the pool side. The spa runs on a shorter clock.
In what order should you balance hot tub chemicals?
Balance total alkalinity first, then pH, then calcium hardness, then sanitizer, and shock last. Alkalinity is the foundation because every acid or base you add afterward moves it. Fixing pH before alkalinity means fixing pH twice. Sanitizer goes in last because it only works properly once pH is in range.
Alkalinity and pH are tied together, and that is where beginners get tangled. Alkalinity is the water’s resistance to pH change. If it is low, pH swings with every bather and every dose, and you chase it forever. If it is high, pH drifts upward and refuses to come down. So test alkalinity, bring it into 80 to 120 ppm with an alkalinity increaser (sodium bicarbonate) or a pH decreaser (sodium bisulfate, which lowers both), run the jets for 20 minutes, and retest before you touch anything else.
Then pH. Spa water wants to climb. Jets and air injectors blow off carbon dioxide constantly, and losing CO2 pushes pH up, which is why a hot tub that was 7.4 on Monday reads 8.0 by Thursday with nobody touching it. Hold it at 7.4 to 7.6 with small doses of pH decreaser. Small is the word. A capful in 400 gallons does what a cup does in a pool, and overshooting low is worse than running slightly high, because acidic water attacks the heater element, the one part of a spa nobody wants to replace.
Calcium hardness only needs setting once per fill, because it does not consume itself the way sanitizer does. Set it once alkalinity and pH are stable and it stays there until you drain. Then sanitizer, then a non-chlorine shock. The spa balancers on our shelves are labeled by the number they move, which makes this easier than it sounds.
Bromine or chlorine: which sanitizer belongs in your hot tub?
Bromine suits most hot tubs because it stays active in hot water and above pH 7.6, where chlorine weakens, and it does not gas off as fast under a cover. Chlorine in dichlor form acts faster and costs less but needs dosing after every soak. Pick one and stay with it; do not mix the two in a floater.
Bromine is the forgiving choice. Tablets in a floater dissolve slowly, the bromide they leave behind builds a reserve in the water, and a weekly non-chlorine shock turns that reserve back into working bromine. It holds up at 104 degrees and stays effective if pH creeps toward 7.8, which pH in a spa always tries to do. The trade-off is that bromine is slower to establish on a fresh fill. Add sodium bromide first to build the bank, then start the floater.
Chlorine for a spa means dichlor granules, and only dichlor. Trichlor pucks are for pools. They are too acidic and too concentrated for a small acrylic shell, and we have seen more than one spa come in with a bleached patch under the floater from someone using pool pucks. Cal-hypo shock is out too, since it adds calcium. Dichlor dissolves fast and sanitizes fast, but it burns off quickly in hot water, so it wants a small dose after every use, and it adds cyanuric acid with every scoop. Once stabilizer builds up the chlorine works slower, and the only cure is fresh water. Our post on cyanuric acid and stabilizer levels explains the chemistry; in a spa the practical rule is that dichlor shortens the time between drains.
Mineral cartridges reduce how much sanitizer you need. They do not replace it; a spa run on minerals alone is a spa that will grow something. Saltwater spas make their own sanitizer, but the water still needs the same four numbers held in range. We covered that option in the hot tub buying guide.
What does Cincinnati water do to a fresh fill?
Greater Cincinnati Water Works delivers water at roughly 125 to 140 ppm calcium hardness, below the 150 ppm floor for a spa, so a city-filled hot tub usually needs calcium added to stop foaming and protect the heater. Well water in Clermont, Brown, Warren and the Northern Kentucky counties often runs hard, with iron, and may need diluting or a metal sequestrant instead.
This is the part no national spa guide will tell you, because it depends on where your hose is connected. City water here is treated and moderately soft. Fill a spa from a Cincinnati, Blue Ash, Mason or Northern Kentucky municipal tap and the calcium reading lands around 130 ppm. That is fine for a pool and slightly low for a spa, and low calcium in a hot tub is the single most common cause of the foam that owners blame on soap. Soft water foams under aeration. The fix is a calcium increaser on the fresh fill, brought up to about 200 ppm, and the foam usually stops the same day. We wrote about the pool version of this in our calcium hardness guide, where the answer is the same: around here you are adding calcium, not fighting it.
Well water is a different animal. Out past the city lines in Clermont and Brown counties, the rural stretches of Warren and Butler, and a good part of Boone, Kenton and Campbell in Kentucky, spas get filled from wells that can run well above 250 ppm hardness and carry dissolved iron or manganese. Those fills scale the heater and stain the shell tan or brown when the sanitizer oxidizes the metal. If you are on a well, test the fill water before it goes in the tub. A hose-end prefilter takes out most of the metals, a sequestrant dosed at fill keeps the rest in solution, and filling partly from a softened tap brings very hard water into range. Either way, the first thing we ask when a spa sample comes to the counter is where the water came from. Bring a cup of the fill water with the spa water and we can test both.
How do you start up a hot tub after a fresh fill?
Fill through the filter well to avoid an airlock, heat the water to at least 90 degrees before testing, then balance alkalinity, pH and calcium in that order with the jets running. Add a sodium bromide bank if you are using bromine, start the sanitizer, shock, and leave the cover open 20 minutes. Retest the next day.
Put the hose in the filter compartment, not the footwell, so the pump primes without trapping air. Once the tub is full and the heater is running, wait for the water to reach at least 90 degrees. Cold water tests differently and dissolves chemicals slowly; the readings you take at 60 degrees will not be the readings you get at 102. Then work through the four numbers in order, retesting between each one.
With the water balanced, build the sanitizer. For bromine, dissolve sodium bromide per the label, then load the floater with tablets and open it about halfway. For chlorine, add dichlor to reach 3 ppm. Either way, follow with a dose of non-chlorine shock (potassium monopersulfate) to oxidize the manufacturing residue in a new spa or the lotion and dust in a refilled one, and leave the cover propped for 20 minutes so the gases clear instead of attacking its underside. The test strips we carry read all four numbers plus bromine and chlorine on one strip. Then write down the date. Everything about spa water care runs on how old the water is, and nobody remembers.
What should a weekly hot tub water routine look like?
Test sanitizer and pH two or three times a week and before every soak, adjust with small doses, shock once a week or after a heavy-use night, rinse the filter cartridge every two weeks and soak it in filter cleaner monthly, and wipe the waterline and the underside of the cover when you shock. Fifteen minutes a week, most of it waiting.
Dip a strip before you get in, because you are checking sanitizer for your own sake, and again a day or two later. Bromine in a floater holds a fairly steady level; chlorine users add a small scoop of dichlor after each use, which is the only way to keep free chlorine from hitting zero overnight after a soak. Check pH at the same time and nudge it if it is above 7.8. Shock weekly with non-chlorine oxidizer, and if you had six people in on Saturday, shock Saturday night rather than waiting for the scheduled day.
The filter is the part people forget, and it is half of water quality. A spa cartridge catches body oils, lotion and the fine particulate the sanitizer has already killed, and once it loads up the water goes dull no matter what the chemistry says. Hose it out every two weeks, soak it overnight in cartridge cleaner once a month, and keep a second cartridge so the spa is never running without one. A cartridge past a year of this is done, however clean it looks. The same goes for the cover: a waterlogged cover drips what it has absorbed back into the water, and as we explained in our post on what drives hot tub running costs in an Ohio winter, it is also costing you heat.
Why does hot tub water go cloudy or foamy when the numbers look right?
A spa can test perfectly and still be cloudy because the test strip does not measure total dissolved solids, body oils, lotion or detergent residue, which accumulate with every soak. Foam nearly always means low calcium, soap residue from suits, or water past its useful age. A clarifier buys a week or two; a drain and refill fixes it.
This is the sample we see most in late winter: pH fine, alkalinity fine, bromine fine, water that looks like weak tea and foams when the jets come on. The chemistry is right. The water is simply full. Every soak adds sweat, skin, lotion and a little laundry detergent from swimsuits, and every dose of every chemical adds dissolved solids that never leave. Sanitizer kills the bacteria in that soup but does not remove the soup. After three or four months no amount of balancing brings it back. The manufacturer rule of thumb is gallons divided by three, divided by the number of people who use it on an average day, equals days between drains. A 400 gallon spa used by two people works out to a refill about every two months.
Before the water is old, the causes are easier. Low calcium foams, and city fills start low, so check that first. Suits washed in detergent carry it into the tub; rinse them in plain water instead. Lotions and hair products go in with every bather, and a scum absorber or a quick wipe of the waterline takes care of a surprising amount of it. Cloudiness without foam is usually a tired filter or a sanitizer level that hit zero overnight after a crowd, and a shock plus a filter rinse clears it in a day. If the water clouds every time you add a specific product, stop adding it and bring the bottle in with your sample.
What changes about spa water in an Ohio winter?
Winter use is heavier, top-off water is colder and shifts the balance, and a cover that stays closed for days lets pH and sanitizer drift unseen. Keep the spa heated and running all winter, test twice a week even when nobody has used it, top off in small amounts, and never let the water sit unheated in a hard freeze.
A Cincinnati winter puts the spa to work in the months a pool sits under a cover, and the chemistry follows the use. More soaks mean more sanitizer demand, and cold dry air pulls water out of an open spa fast, so you top off more often. Every gallon of top-off is unbalanced city or well water, nudging calcium and alkalinity a little each time. Small top-offs, tested after, keep that in hand. Snow that gets in when the cover is off does the same in the other direction, diluting everything at once.
The bigger winter mistake is leaving a spa idle. A hot tub nobody is using in January still needs to be heated and circulating, because the plumbing in the cabinet freezes first, and a split manifold under an acrylic shell is a spring repair nobody enjoys. If you are going away, leave it on, set the temperature down a few degrees, and have someone dip a strip once a week. Draining a spa for winter here is only safe if it is blown out and antifreezed properly, and that is a service call, not a garden-hose job. Our service department handles the winter maintenance and the repair when it goes wrong.
The mistakes we see at the counter
Sixty years of testing water gives you a short list. Pool trichlor pucks in a spa floater, the most common and the most expensive, since the acid etches the shell and bleaches whatever the floater touches. Shocking with the cover closed, which chews up the cover’s underside and does nothing extra for the water. Adding pH decreaser and alkalinity increaser in the same hour, which is pressing the gas and the brake together. Dosing by the pool bottle’s directions, written for 10,000 gallons, into a 400 gallon spa. Running on minerals with no sanitizer because someone said it was chemical-free. And the one that is not chemical at all: keeping the same water past four months because it still looks clear, then wondering why nothing balances. None of these are hard to avoid once you know they exist.
When should you bring the water to us instead?
Bring a sample to the Reading Road store when the strip says one thing and the water says another, when you are on well water, when a new product changed the water, or any time a rash, an odor or a stain shows up. The store’s test reads more than a strip can, and the advice comes with the exact product to fix it.
Test strips are good enough for the weekly routine. They are not good enough for diagnosing a problem, because they read six things and a spa can go wrong on a dozen. The water lab at our store on Reading Road tests calcium, metals, total dissolved solids and stabilizer along with the basics, and the person behind the counter has seen the same cloudy sample a few hundred times. Bring a cup of spa water in a clean container, filled from elbow depth, with a photo of the products you are using, and you will leave with a short list and the right bottles. No appointment. If the spa needs a drain rather than a chemical, we will say so, because selling you a clarifier for water that needs replacing helps nobody.
If you are still on the shopping side of this, the water care conversation is part of every hot tub sale on our showroom floor. Ask about it before you buy. A spa you know how to keep clear is a spa you will actually use in February, and the health benefits only show up in water you are willing to get into. Contact us with a question or a sample any day the store is open.
Frequently asked questions
On a fresh fill you need an alkalinity increaser or pH decreaser to set alkalinity and pH, a calcium increaser if the fill water is soft (Cincinnati city water usually is), a sanitizer (bromine tablets with a sodium bromide starter, or dichlor chlorine), and a non-chlorine shock. A metal sequestrant is worth adding on well water. That is the whole startup list.
At spa temperature, not more than a day or two. Warm water with no sanitizer grows bacteria within hours, and a spa left unsanitized for a week will need a shock, a filter clean and often a drain to recover. If you are going away, keep the sanitizer feeding through a floater and have someone test it weekly.
Because balance tests do not measure what has built up in the water. Body oils, lotion, detergent from suits and the dissolved solids left by every chemical dose all accumulate, and after three or four months the water is full. A clarifier and a filter clean help briefly; the real fix is a drain and refill.
The felt on a tennis ball soaks up body oils and lotion floating on the surface, which is what causes most foam and scum lines. It works, within limits. A purpose-made scum absorber does the same job and lasts longer. Either one is a supplement to sanitizer and shock, not a replacement for them.
Every three to four months for a typical family spa, sooner with heavy use. The manufacturer rule of thumb is gallons divided by three, divided by daily bathers, equals days between drains. When it is time, drain it fully, wipe the shell, clean or replace the filter, and start the four-number balance from the beginning on the new water.