Hot Tub Chemistry 101: Everything You Need to Know

Michael Keenan


Hot Tub Water Chemistry

While a hot tub can offer the ultimate relaxation, as a hot tub owner, you know that it is vital to keep the water clean and safe. You can do this by ensuring that the chemistry of your hot tub water is balanced.

This not only helps to keep the water in good condition but also ensures that all the hot tub equipment is functioning efficiently.

In this article, we’ll discuss all you need to know about hot tub chemistry, how different chemicals can affect the chemistry of the hot tub water, and how you can keep your hot tub functioning at its best all the time. You’ll also find links to other more in-depth articles on this site.

hot tub chemistry

Quick Answer:

Knowing about and managing the chemistry of a hot tub is essential to keep it clean and safe. It’s important to use sanitizers like chlorine, biguanide, and bromine. Test the water regularly, using shocks once a week, balancing alkalinity levels between 100-150 ppm and pH levels between 7.4-7.6. All these steps will help to keep your hot tub functioning at its best all the time!

Understanding Hot Tub Chemistry

You don’t need to be afraid of chemistry when it comes to managing your hot tub. The range of specialist products made for hot tubs and spas has made things easy. There is a hot tub product for every need – all with their own detailed instructions.

But, it is still good to have a working knowledge of the important stuff!

Warm water in a big tub, used by a number of people over a long period of time. What could go wrong? Keeping your spa water in tip-top condition is important if you want to avoid making users sick or spreading skin conditions!

Using chemicals to balance the water in the hot tub can help to keep the water clean and also prevent any bacteria from breeding in the water.

So, it is very important for a hot tub owner to monitor the chemical balance of the water in the hot tub and know how to make changes in the chemistry.

Are Hot Tub Chemicals Necessary?

This is a commonly asked question and people wonder if it is absolutely required to use chemicals in their hot tubs. The simple answer is yes!

The first thing to understand is that the water in the hot tub is not filled and drained as you would do in a bathtub. The water remains in the hot tub for several weeks or even months.

Simply removing leaves, insects, and other debris and vacuuming your hot tub is not enough. Without using chemicals, your hot tub water will become a breeding ground for bacteria, viruses, and other harmful pathogens that can be quite hazardous to health and can cause many hot tub-related illnesses.

It can also result in equipment failure. So, it is important to use the right type of hot tub chemicals to prevent all problems.

But you may be confused about which chemicals to use, how much to add, and how to add them. While all this may seem confusing, there’s no cause to panic.

If you understand the basics of hot tub chemicals, your hot tub maintenance will become a breeze.

The easiest way to determine what chemicals must be added to your hot tub and how much to add to achieve the optimal balance is to test your water. You can do this at home by using a liquid test kit or test strips.

Alternatively, you can carry a sample of the water to your local pool store and get them to check it. Once you have the readings, then you can start adding the proper amounts of chemicals to the hot tub water as required.

spa chemicals

What Chemicals Do I Need For a Hot Tub?

Next up, I’ll cover the main chemicals that are used in hot tub maintenance. You don’t need all these chemicals but at some point, you’ll find most of them useful.


The most important hot tub chemical is a sanitizer, which will help to keep the water sanitized and clean. You must decide which sanitizer to use from the various options such as:

  • Chlorine
  • Biguanide
  • Bromine
  • Salt System
  • Mineral System

Each of these sanitizers has pros and cons, so it is important to understand how each works before deciding which one to use.


The most commonly used sanitizer, chlorine is a powerful sanitizer that can kill bacteria, fungus, viruses, and algae effectively.

It is easy to add and is cost-effective too. However, when you use chlorine, it produces a strong smell, which is basically the smell of the chloramines, produced when the oxidation process of chlorine occurs and breaks down the contaminants.

If you start getting the “chlorine smell” from your hot tub, this can mean that the chlorine has been used up and there is not sufficient chlorine in the water. It also happens just after adding chlorine or chlorine shock to your hot tub.

So, you must test the water and if the chlorine level is below the recommended level, then you must shock the hot tub water to remove the chloramines and bring the chlorine to the level where it starts working again to remove the contaminants effectively.

The ideal chlorine level in a hot tub is around 1 ppm (parts per million) to 3 ppm.


Essentially a non-chlorine-based sanitizer, biguanide helps to kill bacteria and other microorganisms in the hot tub water.

The benefits of using biguanide are that it is odorless, you don’t need to use it very frequently, like chlorine and bromine, and using biguanide helps to make the water feel much smoother.

However, the drawback of using biguanide is that it is more expensive compared to chlorine and bromine.

Also, you need to completely switch your sanitizing system and biguanide can cause the plastic components and rubber gaskets of your hot tub to deteriorate quickly.

The optimal biguanide level for hot tubs is 30 ppm–50 ppm.

Baqua Spa 88855 Sanitizer Chlorine-Free Cleaner for Spas and Hot Tubs, 16 fl oz
  • BENEFITS: Effective bacteria control; Protects against staining and discoloration due to metals
  • USE: Part of BAQUA Spa’s bromine-free and chlorine-free maintenance product line; Use once per week for maximum effectiveness
  • COMPATIBILITY: Works with all biguanide sanitizer systems
  • FEATURES: Chlorine free; Step 3 of BAQUA Spa System- Sanitizer


After chlorine, bromine is the next most popularly used sanitizer for hot tubs. The main advantage of using bromine is that, unlike chlorine, it does not oxidize the contaminants and so, no chloramines are produced.

Bromine works by ionizing the contaminants in the water and breaking them down at the molecular level, producing bromamines as a waste product.

While bromamines don’t smell as unpleasant as chloramines, they reduce the effectiveness of the sanitizer and by keeping the bromine at the appropriate level, you can prevent this from occurring.

Bromine is available in the form of tablets or granules and is easy to use while being less harsh on the skin.

Some of the advantages of using bromine are that it lasts longer compared to chlorine, it is much gentler on the skin and it has lower pH compared to chlorine, enabling you to maintain the balance of the water better.

However, bromine can have some disadvantages.

For one, it works much more slowly compared to chlorine, meaning the contaminants remain in the water much longer before being destroyed.

Bromine is not very stable, so if you use it in an outdoor hot tub that is not covered, it will break down very quickly when exposed to the sun.

Bromine does not have chemicals that it can be stabilized with and therefore, it is best to be used for indoor hot tubs.

The optimal bromine level is around 3 ppm–5 ppm.

Saltwater Systems

Just like saltwater pools, you can also have a saltwater hot tub, which essentially makes use of a saltwater chlorinator to convert the salt into chlorine, which sanitizes the water.

You need to purchase salt, which is quite inexpensive compared to chlorine. Also, the water is gentler and softer on your hair, skin and doesn’t irritate the eyes.

The ideal salt level is around 2,500 ppm although you can maintain a level between 2,000 and 3,000 ppm. The ideal chlorine level is between 1 to 3 ppm and you can measure both the chlorine levels, as well the salinity of the water using saltwater test strips.

Mineral Sanitzizer Systems

Minerals like silver and copper are often used as hot tub water sanitizers because of their bactericide and algaecide properties. The biggest advantage of using a mineral sanitizer is that it helps to reduce chlorine usage and so, the water is less harsh on the skin.

When using minerals, you won’t have issues with chloramine production and problems with the chlorine smell. Minerals are fairly inexpensive and easy to use. There is no need to measure the chemicals to add to the water.

Typically, minerals come in a stick with automatic dosing and all you need to do is add this to your hot tub filter and replace it once every 3-4 months.

However, just using minerals will not help to sanitize the water effectively and you will need to use a bit of chlorine to get the optimal effect. The optimal mineral level in hot tub water is around 0.5 ppm of chlorine or 1 ppm of bromine.

Nature2 Zodiac W20750 SPA Stick Mineral Sanitizer, 1-Pack, Yellow
  • chlorine way to keep your spa free from harmful bacteria
  • Reduces need for harsh chemicals; less skin irritation
  • Works with chlorine or Cense

Hot Tub Shock

While the levels of sanitizer in your hot tub may be at the optimal level, shocking the hot tub once a week can help to deal with water problems such as cloudy hot tub water, foamy water, green algae growth, etc. by raising the sanitization levels.

The main reasons to shock the water using either chlorine or non-chlorine shock in your hot tub are:

  • It helps to remove organic contaminants, especially after heavy use of the hot tub.
  • It kills bacteria (only if you’re using chlorine shock).
  • It reactivates bromides, thereby making them more effective in killing bacteria.
  • It helps to create more free chlorine and bromine, enabling your sanitizer to work better.

The type of hot tub shock depends on the type of hot tub you have:

  • Chlorine-Based Hot Tubs: Chlorine shock, non-chlorine shock or more chlorine.
  • Bromine-Based Hot Tubs: Non-chlorine shock or more bromine.
  • Biguanide Hot Tubs: These systems require shocks specifically designed for the particular brand you’re using.
  • Mineral Hot Tubs: It is better to use non-chlorine shock with mineral systems to keep the levels of chlorine very low.

Irrespective of the type of sanitizer being used in the hot tub, you must ensure that the alkalinity and pH of the hot tub water are balanced. This is important for the sanitizer to work efficiently. You can actually over shock your hot tub and please wait before using the water again. Always test after any chemical treatment.

Alkalinity Levels

The optimal alkalinity level of hot tub water is between 100 to 150 ppm. Total alkalinity (TA) lower than this level will cause corrosion, while if it is higher, then the water will turn cloudy.

The TA level of your hot tub water must be adjusted first before balancing the pH level as the alkalinity acts as a buffer and prevents the pH level from changing drastically.

If the alkalinity of the water falls below 100 ppm, then the pH level will also drop, making the water acidic. So, it is important to keep an alkalinity increaser like sodium bicarbonate at hand.

However, if the alkalinity of the water becomes too high, you can lower it by using a pH decreaser, and finding the right balance of chemicals to get both the pH and alkalinity levels right can be quite tricky.

See my detailed guides on how to lower alkalinity, ways to increase alkalinity and what causes low alkalinity in a hot tub. You’ll be an expert on hot tub alkalinity levels!

pH Levels

The pH levels of the hot tub water must be balanced properly. The optimal pH level is between 7.4 and 7.6. If the pH levels increase above 7.6, then the water will become cloudy and you may have to use a pH decreaser such as muriatic acid or sodium bisulfate to reduce the pH levels.

If the pH levels are too low, then using a pH increaser like sodium carbonate may also cause the alkalinity of the water to increase and you will have to test the water and adjust the levels.

SpaGuard pH Increaser 1.12lb
  • Protects corrosion of equipment due to low pH
  • Balances water for bather comfort
  • Protects your spa or hot tub from corrosion due to low pH

If you find that the pH levels, as well as the alkalinity levels, are low, you can simply add an alkalinity increaser to the water, which will help to raise both levels. And, if the pH, as well as alkalinity levels, are very high, then you can use a pH decreaser to reduce the levels.

However, when you are adjusting the levels of pH and alkalinity, then make sure to test the water multiple times so that you’re able to achieve the right level.

Maintaining the right pH and alkalinity levels is very important because if these levels are low, then the hot tub can get damaged as the water is too acidic.

On the other hand, high alkalinity and pH levels cause scaling, which will also damage the hot tub equipment, putting the heater at risk.

The low pH causes an increase in acidity, which will corrode the heater element, while the high pH deposits scale on the element, which makes it hard for the heater to heat the hot tub.

ph levels

Calcium Levels

The amount of calcium in the water indicates the hardness of the water and it is always recommended that the hardness of the hot tub water should be a bit high because if the water is too soft, it will corrode the shell, pipes, and other components of the hot tub over time.

The ideal calcium hardness level is 175 to 250 ppm.

And, if the calcium level becomes too high, it will cause the water to turn cloudy and the calcium deposits can cause problems for the hot tub shell, element, and equipment. See my guide on how to get rid of calcium flakes in a hot tub for more information.

If the calcium level is not correct, it is best to drain the hot tub, clean it thoroughly by scrubbing it and refill it with fresh water. You can use a calcium hardness increaser to maintain the balance of your hot tub.


Hot tub enzymes are essentially biological catalysts that help to speed up the chemical reactions in your spa. They break down and eliminate organic contaminants from the hot tub water such as sweat, body oils, skin cells, cosmetics, hair products, lotions, insects, leaves, twigs, etc., which can build up over time and produce scum in the hot tub that can be difficult to clean.

Hot tub enzymes help to reduce the amount of sanitizer needed to sanitize and maintain the hot tub.

Water Clarifier

A water clarifier helps to keep the hot tub water clear. It works by attracting the tiny dust and dirt particles and enlarging them so that the filter can capture them.

However, a clarifier is a temporary measure to clear the water and once the effect of the clarifier wears off, the water will return to its earlier state.

You may find a clarifier useful if you need to use the hot tub and don’t have the time to test and balance the water. You can simply pour in some clarifier and clear the water for a short while and deal with the larger issue later.

Clorox® Pool&Spa™ Swimming Pool Super Water Clarifier, Creates Crystal Clear Pool Water, 1 Quart (Pack of 1)
  • Concentrated pool clarifier to clear dull, hazy or cloudy pool water
  • Creates crystal clear pool water
  • Combines Small particles into large ones for easy filter removal
  • Use weekly to maintain clear water

Plumbing Cleaner

A plumbing cleaner helps to flush out the scale deposits, biofilm, oily residue, grime, and other organic, mineral, and chemical contaminants from the hot tub’s plumbing, thereby prolonging the life of both your hot tub and its components.

Water Treatment and Conditioner

Water conditioners help to maintain the hot tub easily by preventing scale build-up, eliminating scum lines, and also maintaining the optimal pH levels while making the water of the hot tub feel soft and smooth.

It also helps to reduce the number of harsh chemicals you would require to maintain the hot tub.

How to Add Chemicals to Your Hot Tub Properly

Adding chemicals to your hot tub water is a simple process and you can do it by simply following the steps described below:

  • The chemicals you add to the water must mix in well and keeping the hot tub running, will enable the hot tub jets to mix the chemicals into the water properly.
  • Once you have added the chemicals, allow them to off-gas or break down and the chemical gases to disperse. So, avoid covering up your hot tub immediately as this will trap the gases. If your hot tub is located outdoors, then the gases will simply disperse into the air; however, if it is indoors, then it is recommended that you turn on a ventilation fan or/and open out a window to allow the gases to disperse.
  • While off-gassing is good, you want the chemicals to remain in the hot tub water and not completely dissipate. So, make sure you turn off the air valves, which increase the jet pressure because too much air in the water will cause the chemicals to off-gas quickly.
  • Before you start adding the chemicals to the hot tub water, you must determine what the water requires and how much. You can check this by testing the water by using a liquid test kit or test strips.
  • Check the manufacturer’s instructions for each chemical and determine how much of each chemical is required to ensure that the water has the correct chemical balance.
  • Once you have figured out what chemicals are required and how much is needed, measure them carefully because you want that the water should be balanced properly and don’t want a chemical overload.
  • The final step is to pour all the chemicals into the hot tub water and allow the hot tub jets to mix them up properly. Allow it to mix for around 10 to 15 minutes and then cover the hot tub with the lid so that the chemicals do not evaporate.
  • Before getting into the hot tub, make sure to test the water to ensure that the chemical levels are at the optimal levels, and if they are not, then you may have to repeat the entire process until the optimal balance is reached.
luxury hot tub

Tips to Prevent a Dirty Hot Tub

Apart from keeping the hot tub water balanced, there are a few things that you can do to ensure that your hot tub is clean, clear, and safe for use such as:

  • Keeping the hot tub clean and sanitized by using a hot tub cleaner regularly and keeping the water clear and sparkling.
  • Showering every time before getting into the hot tub because the sanitizer that you add to the hot tub can only handle so much and when overburdened, it can become ineffective.
  • The filter of your hot tub works very hard to keep the tub clean by removing bacteria, dirt, grime, debris, etc. in order to keep the water clean, healthy, and safe. So, you must clean the filter on a regular basis so that it can do its job of filtering out all the debris from your hot tub water effectively.
  • Even if you clean the hot tub filter regularly, over time, the filter will wear out and needs to be replaced. So, it is important to keep an eye on the condition of the filter and replace it when required.
  • It is common to see a scum ring around the hot tub, which is usually formed by organic contaminants. Putting in a few tennis balls or hot tub sponges can help to remove the film by soaking up the oil, grime, etc. that cause the hot tub water to become cloudy.
  • If your hot tub is outdoors, it will be exposed to insects, leaves, twigs, etc. and if it is indoors, dust and other debris can get in. So, keeping your hot tub covered when not in use can help to reduce the dust, dirt, etc. getting into it. Also, this means lesser chemical usage to keep the hot tub clean.
  • It is important to drain your hot tub completely and clean it once in 3 to 4 months. When you’re doing this, it’s a good idea to use a flush product to remove any biofilm from the plumbing. Check for leaks. Scrub your hot tub and then refill it with fresh water and you’re all set for a relaxing soak again.

At the end of the season you might need to run through some of my guides: how to winterize your hot tub and how to stop your tub from freezing are a good place to start.

The Final Word

Hot tubs require more maintenance than just removing leaves or debris! It’s important to keep the water balanced with chemicals. Chlorine, biguanide, bromine, and salt systems are all options for hot tub sanitizers, each with its own pros and cons.

You need to balance the alkalinity and pH levels of your hot tub water in order to ensure that your sanitizer works effectively.

So, now that you’ve learned all that there is to know about hot tub chemistry, you’re all set to maintain and take care of your hot tub using the right chemicals so that you’re able to enjoy it safely and in a healthy way for the rest of your life.

Last update on 2024-05-19 / Affiliate links / Images from Amazon Product Advertising API

michael keenan author

Author - Michael Keenan

I'm Michael Keenan the owner and creator of the Outdoor Care Guide. I'm a trained horticulturist with over 30 years of experience in pool care, plant care, and landscape care! Seemed like a good idea to share - I think I can make your life easier and save you some time and money!