People who look after a pool well often struggle with a hot tub, and it catches them off guard.
The chemistry is the same chemistry. What changes is the scale. A 400 gallon tub is about two percent of a typical pool, running twenty-five degrees hotter, with jets thrashing air through it. Everything that takes a week in a pool takes an afternoon in a spa.
Key takeaways
- Volume is the whole story. Four people in 400 gallons is the bather load of 200 people in a 20,000 gallon pool.
- Use your maker's ranges. Hot Spring wants alkalinity at 40 to 120, Bullfrog 80 to 120, Jacuzzi and Sundance 100 to 150, and each manual says to follow its own.
- Never put pool trichlor tablets in a spa. They are far too acidic for that little water.
- Bromine keeps working better than chlorine as pH climbs, but the Pool & Hot Tub Alliance advises against pairing it with ozone.
- Add sanitizer after every soak, not on a schedule. The demand follows the bathers, not the calendar.
Why it moves so fast
Three things, and they compound.
The volume is tiny. Four adults in 400 gallons is the same bather load per gallon as 200 people in a 20,000 gallon pool. Every drop of sweat, every trace of sunscreen, every bit of body oil is landing in a very small amount of water.
The water is hot. Hot Spring's manual says dichlor stays effective substantially longer in 80°F water than at 101 to 104°F. Heat also works against you elsewhere: Caldera's manual notes that scale forms more readily in hotter water, and Hot Spring's that bacteria grow rapidly in warm water when the chlorine is low.
The jets aerate constantly. Aeration drives carbon dioxide out of the water, and losing carbon dioxide pushes pH up. It's the same thing that makes salt pools climb, running much harder.
Put those together and a spa can go from perfect to unusable between Friday and Sunday.
The targets
The ranges belong to your tub's maker, and the makers don't agree. Each manual says to follow its own figures, and if the tub is under warranty, that's reason enough. Here's what each current manual says, in ppm, with the Pool & Hot Tub Alliance's figures for any other tub.
| Sanitizer | pH | Alkalinity | Calcium | |
|---|---|---|---|---|
| Hot Spring and Caldera | Chlorine 1 to 5, ideal 3 | 7.2 to 7.8 | 40 to 120, ideal 80 | 75 to 150, or 25 to 75 with their salt system |
| Jacuzzi, with FROG @ease | Chlorine 0.5 to 1.0 | 7.4 to 7.6 | 100 to 150 | 150 to 250 |
| Sundance, without ClearRay | Chlorine 3 to 4, or bromine 2 to 4 | 7.4 to 7.6 | 100 to 150 | 150 to 250 |
| Bullfrog | Chlorine 3 to 5, or bromine 3 to 6 | 7.4 to 7.6 | 80 to 120 | 150 to 250 |
| Pool & Hot Tub Alliance | Chlorine 2 to 5, or bromine 4 to 6 | 7.2 to 7.8 | 60 to 180, ideally 100 to 120 with dichlor or bromine | 150 to 1,000 |
Alkalinity is where they differ most, from Hot Spring's floor of 40 to Jacuzzi's and Sundance's ceiling of 150. The Alliance's alkalinity fact sheet explains part of the spread: dichlor and bromine push pH down, so it puts their ideal at 100 to 120, while hypochlorite pushes pH up and its ideal is 80 to 100.
Two figures don't square up. Sundance says its range without ClearRay comes from the Alliance, but the Alliance's own bromine fact sheet gives 4 to 6 ppm for spas, not 2 to 4. And Hot Spring's and Caldera's manuals give no bromine range at all, apart from one start-up line in the Hot Spot manual that says 1 to 2 ppm.
Stabilizer only matters on dichlor, which adds it with every dose. Bullfrog's limit for its @ease system is 50 ppm, and Leisure Time's Spa 56 label says not to let it pass 100.
The test log asks who made your tub and checks every test against that maker's ranges, with the Alliance's for any other brand.
Go deeperHow alkalinity and pH actually interactWhy pH is the reading and alkalinity is the thing holding it still.The one number that is not about chemistry
Everything above is about keeping the water clean. This one is about the temperature itself, and it belongs on this page because nowhere else on this site was saying it.
104°F is the ceiling, and it is not a preference. The CPSC's position is that hot tub water should never exceed it. At 106°F, water can raise core body temperature to the point of heat stroke — and the CPSC is explicit that this can be fatal to a healthy adult. A tub whose thermostat has drifted, or whose high-limit has been bypassed by somebody chasing a heating fault, is the realistic way people get there.
100°F is the figure the CPSC calls safe for a healthy adult. It is also cheaper to hold than 102°F, because a cooler tub loses heat more slowly.
Three situations where the ceiling is lower or the answer is no:
- Pregnancy. The CPSC warns that soaking above 102°F can cause fetal damage during the first three months, and Hot Spring's and Jacuzzi's manuals say to keep the water at 100°F or below if you are or might be pregnant. That is below the general limit, and it is worth knowing before rather than after.
- Alcohol, and medicine that makes you drowsy. Hot water on its own causes drowsiness, and drowsiness in a tub is a drowning risk rather than an embarrassment. In the CPSC's first hot tub warning, at the end of 1979, three of the ten hot tub deaths it knew of that year were alcohol-related drownings, in tubs at about 110°F. It also says anyone taking medicine that causes drowsiness, such as tranquilizers or antihistamines, should not use a hot tub.
- Children, and anyone with heart disease, blood pressure problems or diabetes. The makers' manuals recommend lower temperatures for young children and for any soak longer than ten minutes, and tell anyone with those conditions to ask a doctor first.
None of this makes a hot tub dangerous. It makes the thermostat worth looking at occasionally, and it makes a tub that will not hold its set temperature a thing to fix properly rather than work around.
Bromine or chlorine
Both work. They fail differently.
| Bromine | Chlorine | |
|---|---|---|
| Active form at pH 8.0 | 83 percent | 23 percent |
| Once combined with waste | Still sanitizes | A slow sanitizer |
| Odor | Low | Combined chlorine smells pungent |
| On a test strip | Free and combined read as one | Combined can be read |
| Reactivates | Yes, with an oxidizer | No, it is used up |
| With ozone | Not advised as the main sanitizer | No such caution |
Bromine dominates spa sanitizing, the Alliance's fact sheet says, and the first row is the reason. Chlorine's effectiveness falls away as pH rises, and spa pH is always rising. Bromine keeps working while you get around to adding acid.
Bromine also regenerates. Spent bromide sits in the water, and shocking with an oxidizer converts it back into active sanitizer. That's why a bromine tub is shocked to reactivate rather than only to destroy waste. Test strips can't tell you when it's time, because they read free and combined bromine together, so shock on the schedule the product's label gives.
Check bromine against your tub first. The Alliance advises against using it as the main sanitizer alongside ozone, because the two together form bromate, a carcinogen. Jacuzzi's and Sundance's manuals say not to use any product containing sodium bromide, the first half of a two-part bromine system, with ozone or UV. And the makers' lists differ. Caldera's has no bromine at all, Hot Spring's includes a granular bromine but not with its silver purifier, Bullfrog's names granular bromine, and Sundance's tubs take bromine tablets in a built-in feeder.
Chlorine is a good choice if you test consistently. But the form matters enormously.
The dichlor problem, and the fix
Dichlor is the right granular chlorine for a spa. It has one catch: every dose adds cyanuric acid, and CYA never leaves except by draining.
In a pool that's slow enough not to matter much. In a spa the water volume is small, so CYA climbs quickly, and once it's high your chlorine gets progressively less effective at the same reading.
One way out is liquid chlorine, which adds no CYA at all. Hot Spring and Caldera list liquid chlorine (sodium hypochlorite) among their approved sanitizers, so on those tubs you can switch to it as the fill ages, before CYA reaches the limits above. Caldera's manual warns that its pH is very high, so expect to add more pH decreaser. Bullfrog and Sundance name dichlor and bromine but not liquid chlorine, so on those tubs a drain on schedule is what resets CYA. Our hot tub chemical picks cover the products for both routes.
Bromine adds no CYA either, if your tub can take it.
Go deeperWhat CYA actually doesWhy stabilizer protects chlorine, and why too much of it stalls everything.The routine that follows from all this
Test before you get in. Not on a schedule — before use. Demand follows bathers, and a tub used hard on Saturday is a different tub on Sunday.
Add sanitizer after every soak, while the jets run with the cover off. That's when the demand happened, so that's when it needs replacing.
Leave the cover open after dosing, with the jets running. Circulation to mix, and a chance for gases to escape rather than sitting under a sealed lid. Sundance says twenty minutes after a shock or a pH chemical, Hot Spring twenty after a shock, and Bullfrog thirty after a shock, to protect the pillows.
Shock weekly, and after a heavy session. Bullfrog's manual says once a week, and more often with heavy use, and Hot Spring's makes a weekly shock required on tubs with its silver purifier. On chlorine that means destroying combined chloramines. On bromine it means converting spent bromide back into working sanitizer.
Watch pH more than anything else. It's the value that moves fastest and the one that quietly disables your sanitizer while every other number looks fine.
Go deeperThe whole routine, weekly to quarterlyWhat to do each week, each month, and when to drain.Mineral systems and salt spas
Two more options you will be offered, both of which sit alongside a sanitizer rather than replacing one.
Mineral cartridges release chelated copper and silver into the water, which suppresses algae and lets you hold a lower sanitizer level. In a spa the bigger load is bather waste, and minerals do not oxidize anything, so you still shock. Check the label before pairing one with bromine: PoolRx, for one, says its minerals are not compatible with bromine or biguanides. Worth having if you want to run your sanitizer at the low end of its range, not worth having as a way to stop dosing. The same reasoning applies in a pool, where the trade is clearer.
Salt spas generate chlorine from salt, the same way a salt pool does, using a small cell built into the tub. The cell is sized for far less water, and the tub's maker sets the targets around it: Hot Spring and Caldera, for instance, want calcium at 25 to 75 ppm with their salt system, against 75 to 150 without. They genuinely reduce handling of chemicals, which is the real benefit — but the water still needs testing, still needs pH management, and still gets replaced every few months.
Neither changes the fundamentals. You still need a sanitizer residual, you still need to oxidize what bathers leave behind, and you still change the water on schedule.
Ozone and UV
Your tub may have an ozone generator, and some have UV. Both are genuinely useful and neither is a sanitizer. If yours has either, read the bromine caution above before choosing a sanitizer.
They reduce the load on your bromine or chlorine by breaking down waste as water passes through the system. What they do not do is hold a residual in the water where the bathers are. Only sanitizer does that.
So they let you run a little less chemical, not none. A tub relying on ozone alone with no measurable residual is unsanitized water at body temperature, which is exactly the condition bacteria want.
Spa chemistry kit
- Check price
spa chlorine granules
Sized and formulated for a few hundred gallons rather than a pool.
- Check price
granular dichlor
Dissolves fast and sits near pH neutral. The right granular chlorine for a spa.
- Check price
AquaChek Select Connect test strips
Fine for a quick mid-week pH check. Not for decisions that cost money.
Frequently asked questions
What should hot tub chemical levels be?
Whatever your tub's maker says, because the makers disagree. Hot Spring and Caldera: chlorine 1 to 5 ppm, pH 7.2 to 7.8, alkalinity 40 to 120 and calcium 75 to 150. Bullfrog: chlorine 3 to 5 or bromine 3 to 6, pH 7.4 to 7.6, alkalinity 80 to 120 and calcium 150 to 250. Jacuzzi and Sundance want alkalinity at 100 to 150 and calcium at 150 to 250. For any other brand, the Pool & Hot Tub Alliance gives free chlorine 2 to 5 or bromine 4 to 6, pH 7.2 to 7.8 and alkalinity 60 to 180.
Is bromine better than chlorine for a hot tub?
It keeps working better as pH rises, which is why the Pool & Hot Tub Alliance says bromine dominates spa sanitizing. At pH 8.0 its fact sheet puts 83 percent of bromine in its active form, against 23 percent of chlorine. Check two things first. The Alliance advises against bromine as the main sanitizer in a tub with ozone, because the two form bromate, and Jacuzzi's and Sundance's manuals say not to use sodium bromide products with ozone or UV. And use a sanitizer your maker lists: Caldera's list has no bromine at all, while Hot Spring, Bullfrog and Sundance each list a form of it.
Why does my hot tub pH keep rising?
The jets. Aeration drives carbon dioxide out of the water, and losing carbon dioxide raises pH. It is the same mechanism that makes salt pools climb, just faster in a small, heavily aerated tub. Get alkalinity into your maker's range first, because the pH dose depends on it, then bring pH down with a pH decreaser.
Can I use pool chlorine tablets in a hot tub?
No. The trichlor tablets sold for pools are strongly acidic and dissolve slowly. In a few hundred gallons they crash pH, corrode heater components and stain the shell. Use granular dichlor, which dissolves fast and is close to pH neutral, or run bromine.
How long should I wait to use a hot tub after adding chemicals?
Run the jets with the cover open, then test before getting in. Sundance says at least twenty minutes after a shock or a pH chemical, Hot Spring twenty after a shock, and Bullfrog thirty after a shock. The sanitizer's label sets the highest level you may soak at: Leisure Time's Spa 56 label prohibits re-entry above 5 ppm of free chlorine, and Hot Spring's and Caldera's manuals say to use the spa only between 1 and 5 ppm.
