Water treatment: How to keep your hot tub water clean

If you want a safe hot tub experience, it is absolutely essential to have clean and healthy water in your spa. Spa water unfortunately is the ideal temperature for proliferation of a lot of bacteria. These bacteria can cause rashes and other infections, so it is super important that you keep your hot tub water both disinfectant and slightly disinfectant (meaning that any bacteria that appears in the spa is killed instantly).

There are several chemical disinfection methods, each with its own advantages and disadvantages.

Firstly, it is important to know the volume of water in your spa in order to correctly dose your products.

Secondly, and we can never say it enough, you must take a shower before you use the hot tub, because the more dirt and sweat you bring into the spa, the more your disinfectant will react with these residues and be neutralised. When all the disinfectant has been neutralised, bacteria will start to multiply, and that's when the dreaded biofilm starts to appear.

These explanations are here to help you make the best choice between chlorine, bromine, and active oxygen, depending on how often you use your spa, so that you can disinfect your spa thoroughly.

These explanations only include treatments for domestic use.

Chlorine: instantaneous and long-lasting

Chlorine is the most commonly-used disinfectant for hot tub water.

It is more accurate to talk about "free chlorine" (Hypochlorous acid, or HOCl-), which is the active component that you get when you dissolve chlorine granules or tablets in your spa or pool water. Contrary to popular belief, free chlorine has almost no odour. The unpleasant smell you associate with public swimming pools comes from combined chlorine - see below.


  • Chlorine is persistent: it remains active for several days in a clean spa

  • It takes effect immediately and is very effective, even at low concentrations

  • Inexpensive


  • Free chlorine reacts very quickly with contaminants in the water, of which the main one is sweat. This reaction neutralises the free chlorine; it becomes "combined chlorine" (chloramines, for the chemistry nerds out there), which has no disinfectant power, and has the familiar chlorine smell. Combined chlorine can also sting your eyes.

  • Free chlorine is sensitive to pH, and it is half as effective at a pH of 8.0 as at a pH of 7.5. As a general rule, you should maintain a pH of 7.6 or lower if you use chlorine as a disinfectant.

See more tips on chlorine treatment.

Bromine: persistent and almost odourless, but slow-dissolving

Bromine has similar qualities to Chlorine, but a few key differences. Unlike Chlorine, it is only available as tablets, and there is no fast-dissolving form.


  • Not pH sensitive

  • Odourless

  • "Combined" bromine disinfects too, although it is less powerful than free bromine.

  • Recyclable: you can reactivate part of the combined bromine into free bromine by adding an oxydant.


  • The tablets can take several days to dissolve (the exact time varies depending on flow rate over the tablets), and there is no fast-dissolving form like chlorine. This can be a good thing, if the way you use the hot tub is stable and regular, but it can require some practice and anticipation to get used to it.

See more tips on bromine treatment.

Active oxygen / Non-Chlorine Shock: odourless but short-lived

Non-Chlorine Shock or monopersulfate (also marketed as "Active Oxygen") is a powerful oxidising agent that kills bacteria and eliminates residues left behind by people.


  • Odourless

  • Leaves almost no residue (like combined chlorine or bromine) which means less chance of eye or skin irritation

  • Not sensitive to pH


  • Unlike chlorine and bromine, it is not persistent. Its disinfectant effect doesn't last very long

Some people rely on Active Oxygen tablets or Non-Chlorine Shock as an alternative to chlorine or bromine. The problem with this approach is that you will have to add the product very often, because its effect is so short. We don't recommend this unless you know you have strong reasons to avoid both chlorine and bromine, like confirmed allergies.

Active Oxygen is a powerful oxydant, and its effect is short-lived, which makes it perfect as a shock treatment. Speaking of which...

Shock Treatments

Chlorine and Bromine are great treatments because they not only sterilise the water, but they stick around, meaning that your spa water remains slightly disinfectant, so that bacteria can never get a foothold. The downsides of both products are that they react with anything organic, leaving behind residues of "combined" chlorine or bromine (chloramines and bromamines respectively). These residues will build up over time, and they're not nice. Combined chlorine smells bad and will sting your eyes and mucous membranes at high concentrations. Both combined chlorine and bromine can trigger allergies in some people.

Shock treatments are powerful oxidants that partially eliminate combined chlorine and bromine, so that your water stays pleasant. They also disinfect (although that is not their main purpose), and oxidise left-over sweat or other residues. If you use bromine, then the shock treatment will also "reactivate" some of the combined bromine into free bromine. Regular shock treatment is an essential part of your water maintenance if you use chlorine or bromine as your main disinfectant. We recommend that you shock at least once a week, but there is no downside to doing it more often, especially if you use the hot tub a lot.

There are two types of shock treatment: chlorine-based and non-chlorine based. We strongly prefer the ones without chlorine for use in hot tubs.

The problem with using chlorine as a shock treatment

The perfect shock treatment is a powerful oxidant with a short lifespan in the water, which leaves almost no residue. Non-Chlorine Shock checks all these boxes, which is why we recommend it.

There are even more powerful shock treatments, however: a family of chlorine-based products, collectively called unstabilised chlorine. They are very effective, but they must be stored and handled with great care. A common one is Calcium Hypochlorite powder, which reacts violently with many everyday products, giving off a poisonous gas and potentially causing severe burns to your eyes and lungs. It is too dangerous for us to sell online.

You can also use stabilised chlorine granules as a shock treatment, and it is sometimes marketed for this purpose for use in home swimming pools. It does work, and it can be convenient to use the same product as both your regular treatment and your shock treatment, but otherwise it's really not the best product for the job. You will have to add a lot of it to get the desired effect (what pool technicians call "breakpoint chlorination"), and then wait as the chlorine level slowly comes down afterwards. We advise you to use Non-Chlorine Shock instead.


A good way to treat your spa is to use chlorine or bromine as your main disinfectant, then add active oxygen or non-chlorine shock regularly as a shock treatment, to oxidise combined chlorine or bromine, as well as any left-over organic matter, such as sweat.

More information on water treatment

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