Hot tub chemistry isn’t alchemy — it’s a discipline with a clear order: first total alkalinity, then pH, and only then the sanitizer. Swap these three steps around and you’ll chase your readings in circles while burning through far more product than necessary. This guide covers the exact procedure, target values, an approximate dosing table, and a comparison of chlorine, bromine, and active oxygen.
Why the Order Matters: Alkalinity → pH → Sanitizer
The water in a hot tub is a small, hot, and intensively aerated volume — typically 1,000 to 1,500 litres at 36 to 40°C. Any mistake shows up many times faster than in a pool holding dozens of cubic metres. That’s exactly why the dosing order is so important.
Total alkalinity (TA) acts as a buffer that keeps pH stable. Until alkalinity sits within the 80–120 ppm range, pH will keep “bouncing,” and any correction to it will hold for only a few hours. That’s why it’s always adjusted first.
pH comes second. It determines how effective the sanitizer is: above pH 7.8, chlorine’s effectiveness drops to roughly a third of what it is at pH 7.2. Acidic water, on the other hand, corrodes heating elements and seals, while alkaline water forms scale and irritates skin.
The sanitizer (chlorine, bromine, or active oxygen) is added only once the water is properly balanced. Only then does it work at full strength and at the lowest dose needed. Adding chlorine to water with an off-balance pH means paying for a product that largely doesn’t work.
What Values Should Hot Tub Water Have?
Before adding anything, test the water — strips are fine for routine checks, a drop tester is more precise. The values below match the recommendations given, for example, by the US site Swim University in its Hot Tub Chemistry 101 guide.
| Parameter | Target range | When to address |
|---|---|---|
| Total alkalinity (TA) | 80–120 ppm | Always first, before adjusting pH |
| pH | 7.2–7.6 | After alkalinity is balanced |
| Free chlorine | 1–3 ppm | Ongoing — check after every use |
| Bromine (chlorine alternative) | 3–5 ppm | Ongoing — top up via a floating dispenser |
| Water hardness (calcium) | 150–250 ppm | At fill-up, then roughly monthly |
Hardness is often underrated in hot tubs. Water that’s too soft foams and “pulls” minerals out of surfaces and metal parts, while hard water deposits scale on the heater and in the jets. In areas with very hard water, filling through a water softener helps.
Dosing Procedure Step by Step
- Turn on the filtration and remove the cover. Chemicals should always be dosed into circulating water with an open surface, so they dissolve evenly and gases can vent off.
- Test the water and write down the values. Don’t dose anything without testing first — guessing is the most expensive maintenance method there is.
- Balance the alkalinity. Low TA is raised with an alkalinity-plus product (sodium bicarbonate); high TA is lowered with pH-minus. After dosing, let the water circulate for 4–6 hours, then retest.
- Fine-tune the pH to 7.2–7.6 using pH-plus / pH-minus. Again, dose in small steps and measure after a delay — a hot tub’s small volume reacts quickly.
- Check the hardness and raise it if needed with a calcium chloride-based product (lowering it is practically only possible by diluting with softer water).
- Add the sanitizer — chlorine granules dissolved in water, bromine tablets in a floater or dispenser, or active oxygen according to the label.
- Leave the cover open for 15–20 minutes. The water needs to vent; closing the cover right after dosing concentrates the fumes and damages its underside.
Approximate Dosing Table per 1,000 Litres
Dosages vary between manufacturers depending on the concentration of the active ingredient — the table below is meant as a first orientation only; the specific product’s label always takes precedence. It’s better to dose less and add more after retesting.
| Product | Approximate dose per 1,000 l | Effect |
|---|---|---|
| Alkalinity plus | 18 g | Raises TA by approximately 10 ppm |
| pH plus | 10 g | Raises pH by approximately 0.1 |
| pH minus | 10 g | Lowers pH by approximately 0.1 (also lowers TA) |
| Chlorine granules (dichlor) | 2 g | Raises free chlorine by approximately 1 ppm |
| Bromine tablets | 1–2 tablets in the dispenser | Maintains 3–5 ppm, refill once dissolved |
| Shock (dichlor) | 10 g | Rapid boost of chlorine to approx. 5 ppm |
| Chlorine-free shock (MPS) | 20 g | Oxidizes contaminants without raising chlorine |
Practical note: always let the water circulate for at least 20–30 minutes between two different products. Never mix concentrates outside the water — each chemical should go in individually, dissolved directly in the hot tub.
For chlorine granules, always choose dichlor for hot tubs, not the cheaper trichlor made for pools. Trichlor is strongly acidic, dissolves slowly, and in a small, hot volume can damage the shell and crash the pH. Dichlor, by contrast, is nearly pH-neutral and dissolves within moments, making it safe and predictable to dose.
Chlorine, Bromine, or Active Oxygen?
The “chlorine vs. bromine” question plays out differently for hot tubs than for pools, because temperature is the deciding factor. Above 35°C, chlorine breaks down quickly and is more sensitive to pH swings. Bromine stays effective even in hot water and across a wider pH range, but works more slowly. Active oxygen is gentlest on the skin, but on its own it can’t handle heavy use and is usually combined with an activator.
| Criterion | Chlorine | Bromine | Active oxygen |
|---|---|---|---|
| Effectiveness at 36–40°C | Good, but breaks down quickly | Excellent, stable even in hot water | Lower, needs regular redosing |
| Odor | Noticeable (chloramines) | Mild | Practically none |
| Sensitive skin | Can irritate | Gentler than chlorine | The gentlest choice |
| Speed of action | Very fast | Slower, but long-lasting | Fast oxidation, short-lived |
| Sensitivity to pH | High | Low — works across a wider range | Medium |
| Running costs | Lowest | Medium | Higher |
For a family hot tub in everyday use, bromine is the most convenient compromise. Chlorine wins on price and speed of action, while active oxygen suits households with small children or sensitive skin — at the cost of more consistent water monitoring. You’ll find the full range of all three systems in the pool and hot tub chemistry category.
Shock Treatment: When and How
Shock treatment is a burst oxidizing dose that burns off organic contaminants — sweat, cosmetics, skin oils — and breaks down chloramines, the compounds responsible for the typical “chlorine” smell and cloudiness. For a hot tub, it’s recommended once a week, plus always after a larger gathering, after an extended shutdown, or whenever the water loses its sparkle.
Procedure: balance the pH to 7.2–7.6, turn on the filtration, add the shock dissolved in water, and leave the cover open for at least 20 minutes. After a chlorine shock, hold off on bathing until free chlorine drops back below 3 ppm — usually 12 to 24 hours. A chlorine-free shock (MPS) allows bathing after roughly 15 minutes, but it doesn’t disinfect — it supplements regular sanitizing, it doesn’t replace it.
A Weekly Routine That Takes Just a Few Minutes
A well-balanced hot tub doesn’t need daily attention, just consistency. A proven weekly rhythm looks like this:
- 2–3× a week: test pH and sanitizer, make small adjustments. The test takes a minute and saves you from major corrections.
- Once a week: shock treatment and an alkalinity check.
- Once a week: rinse the filter cartridge under running water; once a month, leave it overnight in a cleaning solution. A clogged filter is the most common cause of weak flow — if an FLO error appears on the display after neglected maintenance, you’ll find the procedure in the article Hot Tub Showing an FLO Error: How to Fix It.
- Ongoing: a soap-free shower before bathing significantly cuts chemical consumption — most of the load on the water comes from cosmetics and sweat.
The Most Common Mistakes When Dosing Chemicals
- Everything at once. Products added in a single wave react with each other, and the resulting readings make no sense. Always one step, circulation, retest.
- Chemicals in cold water. Granules dissolve poorly in water below 20°C, sink to the bottom, and can leave stains on the shell. Dose only into heated, circulating water.
- Closing the cover right after dosing. Concentrated fumes degrade the underside of the cover and the headrests. After every dose, let the hot tub vent for 15–20 minutes.
- Overdosing. “More just to be safe” leads to skin irritation, corrosion of parts, and the need to dilute the water. A hot tub’s small volume responds to small doses.
- Dosing without testing. Without numbers, you don’t know what you’re adjusting. Test strips are the cheapest item in the entire maintenance routine.
When Chemistry Is No Longer Enough and It’s Time to Change the Water
Every batch of water has its lifespan. Dissolved solids (TDS) gradually build up through dosing, and beyond a certain point chemistry stops working — the water turns cloudy, degrades quickly after a shock, foams, or smells even at correct readings. For a regularly used hot tub, plan on changing the water every 3 to 4 months. We’ll cover the water change itself, flushing the plumbing, and proper refilling in detail in a separate article.
Frequently Asked Questions
How long after adding chlorine can I get in?
After a normal maintenance dose, roughly 30 minutes, but always only after retesting — free chlorine must be in the 1–3 ppm range. After a shock treatment, wait 12–24 hours; for a chlorine-free shock, about 15 minutes is enough.
Can I switch a hot tub from chlorine to bromine?
Yes, but not just by swapping products. It’s best to combine the switch with a water change and a thorough filter rinse, since leftover chlorine chemistry and bromine shouldn’t be mixed in concentrated form. After that, simply load bromine tablets into the dispenser.
Why does my pH keep drifting even though I adjust it regularly?
The cause is almost always low alkalinity. If TA is below 80 ppm, the water loses its buffering capacity and pH fluctuates with every use of the hot tub. Balance the alkalinity to 80–120 ppm first, and the pH will stabilize.
Is testing the water once a week enough for a hot tub?
For hot, heavily used water, that’s not enough. The recommended minimum is 2–3 times a week, plus a quick check after any larger bathing session. A small volume of water can go out of balance in a single evening.
Is active oxygen sufficient sanitizing for year-round use?
For a lightly used hot tub with disciplined maintenance, yes, usually combined with an activator. With frequent use or more users, bromine is more reliable, or oxygen supplemented with a regular chlorine shock.
The right order — alkalinity, pH, sanitizer — turns hot tub care from trial and error into a few minutes of routine each week. Everything you need, from testers through pH correctors to chlorine, bromine, and the oxygen range, is in the pool chemistry category, and if you’re still choosing a hot tub, browse our hot tub range or check out more articles in the Guide.
