Hot Tub Chemicals Guide

Posted by Nick Clamp in Maintaining a Hot Tub Guides on 11th January 2022

Adding chemicals to a hot tub

You need to keep your hot tub topped up with chemicals to make sure the sanitiser you add works and it stays hygienic.

Read on to find out everything you need to know about hot tub chemistry.

Everything you need to know about hot tub chemistry

“Hot tub chemistry” might sound daunting, but it’s really just a matter of keeping your hot tub topped up with a few different chemicals.

There are three things you’ll be adjusting with those chemicals: Total Alkalinity (TA), pH, and Total Hardness (TH).

Get to grips with these three parts of hot tub chemistry and you will be able to troubleshoot any problems you have with your hot tub’s water, like cloudy or foamy water.

Total Alkalinity

Total Alkalinity (TA) measures the total amount of alkaline in the water. 

Alkaline neutralises acid, so your hot tub’s water will irritate your eyes and skin – and can even erode your spa’s components – if its TA is too low. 

Your hot tub therefore needs an alkalinity level of between 125 and 155 parts per-million (PPM). 

The sanitiser you add to your hot tub won’t act effectively if TA isn’t in this range. Your hot tub’s water might also become cloudy or foamy. 

You can increase TA using an alkalinity increaser, which adds more alkalinity to the water. You’ll bring it down using a pH decreaser, as decreasing the water’s pH will bring the alkalinity down.


Checking hot tub pH levels

As you might remember from school, pH is measured on a scale from zero (acidic) to fourteen (basic), with seven being neutral. 

If your hot tub’s water is too far to either extreme you’ll run into problems, so you need to keep it between 7.2 and 7.6. 

You can use pH increaser and decreaser to change your hot tub’s pH level. 

Because water’s Total Alkalinity affects its pH, you should always adjust your hot tub’s TA first, then its pH.

Total Hardness

Total Hardness is the amount of calcium in water. 

Hard water contains lots of calcium and magnesium, which builds up on taps and in pipes and forms limescale. Some parts of the UK have hard water, while others have soft water (which is lower in calcium and magnesium and high in sodium).

Hot tub water needs to be on the hard side, or else it will gradually eat away at your hot tub’s shell and components over time.

The ideal Total Hardness in your hot tub is 175-250 PPM, and you can adjust this with calcium hardness increaser and decreaser

What hot tub chemicals do you need?

Every hot tub owner needs these chemicals:

  1. Hot tub sanitiser like chlorine or bromine (more on this later).
  2. Alkalinity increaser to raise your hot tub’s Total Alkalinity.
  3. pH increaser and decreaser to control your hot tub’s pH level.
  4. Calcium hardness increaser and decreaser to control your hot tub’s Total Hardness.

Optional chemicals you might also want to pick up include:

  1. Line flush cleaner, which flushes every bit of bacteria out of your hot tub’s pipes.
  2. Filter cleaner, which makes cleaning your hot tub filters a lot easier.
  3. Defoamer, in case you ever need a temporary and short-term fix for hot tub foam.
  4. Hot tub shock, in case you ever need a temporary and short-term fix for cloudy hot tub water.
  5. Hot tub enzymes, which help break down organic contaminants in your hot tub’s water and prevent bacteria from developing.
  6. 303 Aerospace Protectant, which protects your hot tub’s vinyl cover from UV rays, stopping it from fading and cracking.
  7. Water conditioner, which eliminates scum lines, prevents scale build-up, helps maintain proper pH levels, and makes spa water feel soft and silky.

Which hot tub sanitiser is best?

Sanitiser is a key hot tub chemical, as it keeps your hot tub hygienic and safe to use by killing bacteria.

The two most popular sanitisers are chlorine and bromine. Neither is particularly superior, as they each come with their own set of pros and cons:


Chlorine is most home spa owner’s sanitiser of choice.

Chlorine creates a bacteria-killing acid when it’s added to water. This acid leaves behind chloramines, which causes that swimming pool smell.

To keep your hot tub clean, you need to keep its chlorine levels between 3 and 5mg/l (milligrams per litre). Lower than this and it won’t be effective at killing bacteria. Higher and it can damage your spa’s components and irritate your skin, eyes, and lungs.

Chlorine comes as both granules and tablets. You place chlorine tablets in a floating dispenser and they dissolve gradually over time, while you add granules directly to your hot tub’s water every time you need to boost its chlorine levels.


Just like chlorine, bromine dissolves in water and kills bacteria.

Bromine isn’t as strong as chlorine, so you need to add more each time you top it up to keep it at 4 and 6mg/l. But it also acts more slowly, so you don’t need to top it up as often. Plus, it doesn’t create chloramines, so you don’t get that distinctive chlorine smell.

Because it’s a bit kinder on the skin, bromine is a good option if you have sensitive skin.

But it is a bit more expensive and not as readily available as chlorine, which you’ll find in any reputable hot tub dealership across the country.

No matter which kind of sanitiser you go for, be sure to carefully follow the manufacturer’s instructions every time you add it to your hot tub to make sure it works as effectively as possible.

And never use chlorine and bromine at the same time, as this can cause a dangerous reaction. If you want to switch from one sanitiser to the other, drain and deep clean your hot tub to avoid cross-contamination and use the new sanitiser in a fresh batch of hot tub water.

How often you should add hot tub chemicals

You should test your hot tub’s pH and sanitiser level every day:

  1. Add a pH increaser if your hot tub’s pH is below 7.2 and a pH decreaser if it’s above 7.6.
  2. If you use chlorine to sanitise your hot tub, use a chlorine test strip to check the chlorine level is between 3 and 5mg/l. If it’s below this, top it up.
  3. If you use bromine to sanitise your hot tub, use a bromine test strip to check the bromine level is between 4 and 6mg/l. If it’s below this, top it up.

Every week, you should test the total alkalinity and total hardness:

  1. Add an alkalinity increaser if your hot tub’s TA is below 125 PPM and a pH decreaser if it’s above 155 PPM. Then test the pH and adjust accordingly.
  2. Add calcium hardness increaser if your TH is below 175 PPM and a calcium hardness decreaser if it’s above 250 PPM.

How to add hot tub chemicals

You will get the best results if you stick to this routine whenever you add chemicals to your hot tub: 

First, you need to test your hot tub’s water using test strips or a liquid test kit to see how much the Total Alkalinity, pH level, Total Hardness, and sanitiser level need to be adjusted.

Keep your hot tub running, as this will help mix the chemicals you add in. And close your hot tub’s air valves so the jets aren’t running too powerfully, which will cause the chemicals to gas off too quickly. 

Grab the chemicals you need to add, then refer to the manufacturers’ instructions to see how much of each you need to add to get your hot tub’s chemistry right.

Precision is key here, so be sure to carefully measure out each chemical before adding it rather than eyeballing it.

Always start by adjusting the Total Alkalinity (if it needs changing). Once you’ve got that dialled in, check the pH level and adjust that if necessary. Then you should test and adjust the Total Hardness and sanitizer levels. 

Add your chemicals and leave your hot tub’s jets to circulate them for at least 15 minutes. Leave your hot tub’s cover off here so the chemicals have a chance to off-gas.

Before you get in your hot tub, test its levels again to make sure they’re where they need to be and adjust again if necessary.

Can you use your hot tub without chemicals?

If you don’t take care to balance your hot tub’s chemistry or keep it topped up with sanitizer it will become a breeding ground for bacteria – and certainly not something you’ll want to hop into.

At an absolute minimum, you need to add sanitiser to your hot tub and the chemicals needed to keep its Total Alkalinity, pH level, and Total Hardness in check.

How soon can you use your hot tub after adding chemicals?

You need to leave your hot tub sitting with its jets running and the cover off for at least 15 minutes after adding chemicals before you use it. So, be sure to test your home spa’s water and make any adjustments to its chemistry before you’re planning on hopping in.

How long do hot tub chemicals last?

Hot tub chemicals tend to have a shelf life of several years. Be sure to check the packaging to make sure your chemicals are still in date if you’ve had them for a while though. 

How to store hot tub chemicals

Hot tub chemicals can be dangerous, so be sure to keep tucked away in a cool, dry place well out of reach of children. You should also make sure the lid is on tight after each use and mince measurement containers out before and after each use.

How to dispose of hot tub chemicals

Hot tub chemicals could be harmful to the environment, so they need to be disposed of properly.

If you have leftover hot tub chemicals you no longer need, consider giving them to another hot tub owner.

If you can’t find anyone to take your unwanted chemicals off your hands (or they’re out of date), contact your local council for guidance on how to safely dispose of them.

Summing up

So, there you have it: absolutely everything you need to know about hot tub chemistry.

While talk of Total Alkalinity and pH levels might seem daunting at first, all there really is to it is regularly testing your hot tub’s water and adding a few scoops of chemicals if it needs adjusting.

If you ever get stuck, just refer back to this guide to get clear on what you might be missing.

About the author

Nick Clamp

I am the Editor-in-Chief at WhatSpa? Media Group and my job is to keep you informed about the very latest hot tubs on the UK market... the best job in the world! When I'm not being deluged with press releases and hot tub brochures I enjoy keeping fit and participating in endurance events including triathlons and distance running.

Latest articles from our knowledge bank

The UK’s No. 1 Spa & Hot Tub Buyers Guide

This edition is full of spa reviews as well as top tips on buying, owning and maintaining your very own hot tub.

Don’t buy a hot tub or swim spa without it!

Get Your Free Copy