You’ve just filled your hot tub for the first time, torn open the starter kit, and you’re staring at six different tubs and bottles with names that sound like they belong in a chemistry lab. Alkalinity increaser. Sodium dichlor. Non-chlorine shock. Calcium hardness. Your eyes glaze over, you chuck in a random scoop of something white, and hope for the best. Sound familiar?
This glossary exists to fix that. It’s not a step-by-step dosing guide — if you want that, head to our hot tub chemicals for beginners guide instead. What you’ll find here is a plain-English reference for every chemical term you’re likely to encounter as a hot tub owner in the UK. Bookmark it, come back when you hit a word you don’t recognise, and stop pretending you know what “total dissolved solids” means. We’ve all been there.
Sanitisers — The Chemicals That Kill Bacteria
These are the workhorses of hot tub water care. Without a sanitiser, your warm, bubbly water becomes a breeding ground for bacteria within hours — the Health and Safety Executive warns that poorly maintained hot tubs can harbour Legionella and Pseudomonas. Every hot tub needs a sanitiser, running constantly.
Chlorine (Sodium Dichlor)
The most common hot tub sanitiser in the UK, and the one most starter kits include. Hot tub chlorine comes as granules (technically sodium dichloroisocyanurate, but everyone just says “dichlor”). You dissolve a small scoop in the water after each soak to kill bacteria and break down organic contaminants like sweat and body oils.
Target level: 3–5 parts per million (ppm). A 1 kg tub costs around £8–12 from Amazon UK or any pool supplies shop and lasts a couple of months. Don’t confuse hot tub chlorine granules with swimming pool tablets or liquid bleach — they’re different formulations and the wrong one can damage your tub.
Bromine
The main alternative to chlorine. Bromine tablets dissolve slowly in a floating dispenser, providing continuous sanitisation. Many owners prefer it because it’s gentler on skin, produces less of that “swimming pool smell,” and stays effective across a wider pH range.
Target level: 3–5 ppm. Roughly twice the price of chlorine (a 1 kg tub runs about £15–25), but plenty of owners find it worth the premium, especially with sensitive skin.
Chloramines and Bromamines
Not something you add — something you want to get rid of. Chloramines form when free chlorine reacts with nitrogen-based compounds in the water (sweat, urine, cosmetics). They’re responsible for that harsh “chlorine smell” that people associate with over-chlorinated pools. Ironically, a strong chlorine smell usually means there’s not enough free chlorine, not too much. The fix is to shock the water to break chloramines apart and restore the free chlorine level.
Bromamines are the bromine equivalent. The key difference: bromamines retain some sanitising power, unlike chloramines which just smell bad and irritate your eyes. This is one reason bromine systems tend to be lower maintenance overall.
Water Balance — Getting the Numbers Right
These terms relate to the chemical balance of your water. Getting them into the right ranges is what keeps your water comfortable to soak in, protects your hot tub equipment, and ensures your sanitiser actually works properly.
pH
The measure of how acidic or alkaline your water is, on a scale of 0 to 14. Pure water sits at 7 (neutral). Hot tub water should be between 7.2 and 7.6 — slightly alkaline.
Why does it matter? If pH drops below 7.0, the water becomes acidic enough to corrode metal components, eat through rubber seals, and irritate your skin and eyes. If it rises above 7.8, your sanitiser loses effectiveness (chlorine is roughly half as potent at pH 8.0 as at pH 7.2), and you’ll start seeing cloudy water and scale buildup.
I check pH before every soak. It takes ten seconds with a test strip and saves you from problems that are much harder to fix later.
Total Alkalinity (TA)
Think of total alkalinity as the bodyguard for your pH. It measures the water’s ability to resist sudden pH changes — technically, it’s the concentration of alkaline substances (mostly bicarbonates) dissolved in the water. Target range: 80–120 ppm.
When alkalinity is too low, pH becomes unstable and bounces around wildly every time you add chemicals or someone gets in the tub. When it’s too high, pH tends to drift upward stubbornly and the water can turn cloudy. Always adjust alkalinity before adjusting pH, because changing alkalinity shifts pH as well, but not the other way around.
Calcium Hardness
The amount of dissolved calcium in your water. If you live in a hard water area (much of southern and eastern England), your tap water already contains plenty of calcium. Scotland, Wales, and the north-west tend to be much softer.
Target range: 150–250 ppm. You can check your area’s water hardness on the United Utilities water quality map or your local supplier’s website. Too low and the water becomes aggressive — it’ll corrode metal fittings and even etch acrylic shells. Too high and you’ll get calcium scale deposits on the waterline, inside pipes, and on heating elements. Scale on a heater element reduces efficiency and can cause it to fail — an expensive repair.
If your area has very hard water, a hose-end pre-filter (about £15–20 from SpaGuard or similar) removes a fair chunk of calcium before it enters the tub.
Parts Per Million (ppm)
The unit used to measure chemical concentrations in water. One ppm means one milligram of a substance per litre of water. You don’t need to do any maths — just match the number on your test strip to the recommended range for each chemical.
Shock Treatments — The Deep Clean
Shocking is something you do periodically (usually weekly, or after heavy use) to oxidise organic contaminants and restore your sanitiser’s effectiveness. Think of it as resetting the water.
Oxidiser (Non-Chlorine Shock)
The gentler form of shock treatment. Non-chlorine shock (usually potassium monopersulphate, often sold as “MPS” or “oxy shock”) doesn’t add sanitiser — it oxidises organic material, breaking down the gunk that makes water cloudy, smelly, or foamy.
The big advantage: you can use the tub about 20 minutes after dosing, compared to waiting hours after a chlorine shock. Most owners use this weekly, with an occasional chlorine shock for a deeper reset. Expect to pay £10–15 for a 1 kg tub from Amazon UK or specialist suppliers.
Chlorine Shock (Superchlorination)
A larger-than-normal dose of chlorine that raises the free chlorine level to 10+ ppm. This kills bacteria or algae that a normal sanitiser level might miss and destroys built-up chloramines.
You can’t use the tub until chlorine drops back to 5 ppm or below — typically 12–24 hours. Most owners do this monthly or after a party where the tub gets heavy use.
Breakpoint Chlorination
When you add chlorine to water containing chloramines, the chlorine initially combines with them rather than destroying them. You have to add enough to push past the “breakpoint” — the threshold where all chloramines are fully oxidised and any additional chlorine becomes free, active sanitiser again. Half-hearted shock treatments can make the problem worse. If you’re shocking with chlorine, commit to it — add enough to reach 10 ppm and let it do its job properly.
Adjusters — Moving the Numbers Up or Down
These are the chemicals you use when a test shows something is out of range.
pH Increaser (Sodium Carbonate / Soda Ash)
Raises pH when it drops too low. Also called “pH Plus” on most product labels. Dissolve the recommended amount in a jug of warm water before adding it to the tub — adding dry powder directly can cause localised pH spikes that damage surfaces.
pH Decreaser (Sodium Bisulphate / Dry Acid)
Lowers pH when it climbs too high. Also called “pH Minus.” This is the one you’ll use more often — hot tub water has a natural tendency to drift upward in pH, especially when the jets are running and aerating the water. Add it gradually and retest after 30 minutes.
Alkalinity Increaser (Sodium Bicarbonate)
Raises total alkalinity. Yes, this is essentially bicarbonate of soda — the same stuff in your baking cupboard, though the pool-grade version is more finely ground and dissolves better. Some owners do use food-grade bicarb in a pinch, and it works fine, but the purpose-made product (about £6–8 per kg) dissolves more cleanly.
Calcium Hardness Increaser
Raises calcium levels in soft water areas. Not something every owner needs — if you’re filling from a hard water supply, you’ll never touch this. But if you’re in a soft water region and notice your test strips showing calcium below 150 ppm, a calcium increaser protects your tub components from the corrosive effects of hungry, mineral-poor water.
Water Clarity — Fixing Cloudy or Foamy Water
Clarifier
A liquid additive that clumps together tiny particles floating in your water — particles too small for your filter to catch on their own. The clarifier causes them to stick together into larger clumps (a process called flocculation), which the filter can then trap.
Clarifiers are a band-aid, not a cure. If your water keeps going cloudy, the root cause is usually low sanitiser, a dirty filter, or poor water balance. Fix those first. But a dose of clarifier after sorting the underlying issue speeds up the clearing process noticeably. A 500 ml bottle costs about £8–12 and lasts ages — you only use a capful at a time.
Anti-Foam (Defoamer)
Foam on the surface usually means surfactants in the water — body lotions, shampoo residue, laundry detergent from swimwear. A few drops of anti-foam knock it down instantly.
Like clarifier, this treats the symptom. If foam keeps returning, shower before soaking, rinse swimwear without detergent, and do a water change. But for a quick fix when guests are coming over, it does the job.
Biofilm Remover (Pipe Flush)
Biofilm is a slimy layer of bacteria that builds up inside the plumbing — pipes, jets, and pump housing you can’t see or scrub. It protects bacteria from your sanitiser, and bits break off into the water, causing persistent cloudiness or foam that won’t shift.
A pipe flush product (around £10–15) is added before you drain. Run the jets for 30–60 minutes to circulate it through the plumbing, then drain and refill. I do this every time I change the water — roughly every three months — and the difference in water clarity during the first week is noticeable. If you’ve bought a second-hand tub, a pipe flush before your first fill is essential.
Testing — How You Know What to Add
Test Strips
The simplest testing method. Dip a strip in the water, wait 15 seconds, and compare the colour pads against a chart on the bottle. Most strips test for free chlorine (or bromine), pH, and total alkalinity simultaneously. A pot of 50 costs about £8–15 from Amazon UK or Argos.
They’re accurate enough for domestic use. The main pitfall: old strips give unreliable readings. Keep the lid sealed, store them dry, and replace them past the expiry date.
Liquid Test Kit (Phenol Red / DPD)
More accurate than strips. These kits use reagent drops added to a water sample — DPD for chlorine/bromine, phenol red for pH. You compare the colour against a comparator disc. Kits cost £15–25, and the reagent bottles need replacing annually. Worth having when you’re troubleshooting a persistent issue that strips aren’t pinpointing.
Free Chlorine vs Total Chlorine
Your test results may show two chlorine readings. Free chlorine is the active, working chlorine available to kill bacteria. Total chlorine is free chlorine plus combined chlorine (chloramines). If total chlorine is much higher than free chlorine, chloramines have built up and you need a shock treatment. Ideally, both numbers should be close together.
Other Terms You’ll Encounter
Total Dissolved Solids (TDS)
The total concentration of everything dissolved in your water — minerals, chemicals, organic matter. TDS rises over time as you add chemicals and contaminants build up. Above 1,500 ppm, water balance becomes harder to maintain and your sanitiser works less efficiently.
You can’t reduce TDS with more chemicals — the only fix is draining and refilling. This is why hot tubs need a complete water change every 3–4 months, even if the water looks clear.
Cyanuric Acid (CYA / Stabiliser)
Protects chlorine from UV light degradation. Important for outdoor pools but largely unnecessary for hot tubs — your cover blocks most UV, and dichlor granules already contain a small amount.
CYA accumulates over time and doesn’t break down. If it builds too high (above 50 ppm), it inhibits chlorine’s sanitising power — a condition called “chlorine lock” where your readings look fine but the chlorine isn’t working. Yet another reason regular water changes matter.
Sequestrant (Stain & Scale Preventer)
Binds to dissolved metals (iron, copper, manganese) in your water and prevents them from oxidising into visible stains or depositing as scale. If you notice brown, green, or blue discolouration after filling, a sequestrant added during the initial fill prevents metals from causing problems as pH and temperature change.
Ozone (O₃)
Not a chemical you add — some mid-range and premium hot tubs have a built-in ozonator that generates ozone gas and injects it into the water. Ozone is a powerful oxidiser that reduces the amount of chlorine or bromine you need by 50–70%.
An ozonator doesn’t replace your sanitiser entirely, but it cuts chemical usage noticeably. If you’re choosing a new tub, it’s worth looking for — our guide to choosing a hot tub covers which brands include them as standard.
UV-C Sanitation
Similar concept to ozone: a UV-C lamp fitted inline with the plumbing exposes water to ultraviolet light, killing bacteria and algae as they pass through. Supplements your sanitiser rather than replacing it. Less common in domestic UK tubs than ozonators, but gaining popularity in premium models.
Salt Water System (Saltwater Chlorinator)
A system that generates chlorine automatically from dissolved salt in the water. An electrolytic cell converts the salt into chlorine on demand — no more measuring and adding granules after every soak. The water feels noticeably softer too.
The trade-off: salt systems are expensive to retrofit (£200–400+), the electrolytic cell needs replacing every 2–3 years (about £100–150), and you still need to monitor pH and alkalinity manually. Several top hot tub brands now offer salt systems as a factory option, which is the more cost-effective route.
Quick Reference — What Goes Where
Here’s the at-a-glance version of the key target ranges for domestic hot tub water:
- Free chlorine — 3–5 ppm
- Bromine — 3–5 ppm
- pH — 7.2–7.6
- Total alkalinity — 80–120 ppm
- Calcium hardness — 150–250 ppm
- Water temperature — 36–40°C (most owners settle on 37–38°C)
If you’re new to all of this, focus on sanitiser and pH first. Those two matter most day-to-day. Once you’re comfortable with those, total alkalinity and calcium hardness slot in naturally.
Hot tub water care sounds complicated until you’ve done it a few times. After a fortnight of ownership, testing and dosing becomes a two-minute habit. The terminology is the intimidating part — and now you’ve got a reference for that.