Hot tub water sits at close to body temperature, which is perfect for you and just as good for bacteria. A few hundred litres shared by several people needs a sanitiser and a bit of balancing. It sounds like chemistry homework. In practice it takes a few minutes, two or three times a week.
This guide covers general water care. It is not about any one product: always follow the dose and safety instructions on the label of what you buy.
The three jobs
- Kill germs with a sanitiser: chlorine or bromine.
- Keep the pH in range so the sanitiser works and the water does not sting.
- Keep the water clean with a filter and regular water changes.
Chlorine or bromine?
| Chlorine | Bromine | |
|---|---|---|
| How it comes | Granules, usually | Tablets in a floater, or granules |
| Speed | Works fast | Works more slowly |
| In hot water | Breaks down faster above about 30 °C | Holds up better at hot tub temperatures |
| pH sensitivity | Loses strength as pH rises | Less affected by pH |
| Smell | The familiar "pool" smell | Milder |
| Cost | Cheaper | Dearer |
| Usual target | About 3 to 5 ppm free chlorine | About 3 to 5 ppm bromine |
Both work. Chlorine suits you if you use the tub now and then and want low running costs. Bromine suits you if you use the tub often, keep it hot all the time or dislike the smell. The target ranges above are typical. Use the range on your test strip tub and your product label.
Pick one and stick with it. Do not switch back and forth without draining first, and never mix the two in the same container.
pH: the number that makes the rest work
pH runs from 0 (acid) to 14 (alkaline). Aim for 7.2 to 7.6.
- Too low (below 7.2): the water stings your eyes and can corrode metal parts and the heater.
- Too high (above 7.6): chlorine loses strength, the water turns cloudy and scale forms.
You raise pH with a pH increaser and lower it with a pH reducer. Small doses, wait, test again.
Total alkalinity
Alkalinity is a buffer: it stops the pH from swinging every time someone gets in. Most test strips show it, and a common target is about 80 to 120 ppm. Fix alkalinity first, then pH, because changing one moves the other.
Test strips: how to read them
- Dip a strip in the water at elbow depth, away from the jets.
- Take it out and hold it flat. Do not shake it.
- Wait the time on the tub (usually 15 seconds or so).
- Compare each pad with the colour chart on the side.
- Write the numbers down. A pattern over a few weeks tells you more than one reading.
Test before each use for the first few weeks, then two or three times a week once you know how your tub behaves.
Shock
"Shock" means a larger dose of oxidiser that burns off sweat, skin cells, cosmetics and other gunk the sanitiser has bound up. Shock once a week, or after a busy session. Leave the lid off for the time the label says, so the gases can escape.
Hard water
In limescale areas, scale builds up on the heater and the walls. A scale inhibitor helps. Three Intex tubs in our data list a built-in hard water system: the PureSpa Bubble 8, Jet & Bubble Deluxe and Greywood Deluxe 4.
Filters
The filter catches what the chemicals do not. Rinse the cartridge every few days of use with a hose, and replace it when it stays grey after rinsing. A clogged filter strains the pump and can stop the heater. The Canadian Spa Co. Grand Rapids lists Microban filters.
Extras that help
Three MSpa tubs in our data have a UVC sanitiser: the Tribeca, Denver and Aero Plus. UV light helps keep water clear between changes. It does not replace your chlorine or bromine.
A simple routine
| When | What |
|---|---|
| Before each use | Quick test strip, top up sanitiser if low |
| Two or three times a week | Full test: sanitiser, pH, alkalinity |
| Every few days of use | Rinse the filter |
| Weekly | Shock |
| Every 1 to 3 months | Drain, clean and refill |
Safety rules
- Add chemicals to water, never water to chemicals.
- One chemical at a time, with the pump running, and let it mix before adding the next.
- Never mix products in a cup or scoop.
- Store them dry, cool, apart from each other and away from children and pets.
- Shower before you get in. Less sweat and lotion means less sanitiser used.
Quick answers
Why does my tub smell of chlorine? A strong smell usually means used-up chlorine compounds, not too much chlorine. Shock the water.
Cloudy water? Check pH and alkalinity first, then shock and rinse the filter.
Where can I drain treated water? Not into a pond or onto a vegetable bed. See where to put a hot tub.
More on filters and winter care in hot tub water care and winter use, and on cold-weather running in using a hot tub in winter.
