A Beginner’s Guide To Hot Tub Chemistry

You definitely don’t need a science degree to keep the water in your hot tub balanced, but hot tub chemistry is the most important thing you can learn in order to keep your water clean and safe for its soakers.

I mean, who wants to sit around in a tepid pool of filth and bacteria, right?

Aside from making sure your water is clean, keeping the water chemistry balanced also helps prolong the life of your spa and equipment.

And let’s face it: all that stuff didn’t come cheap, so you would probably like to do everything you can to make it last.

So, let’s just dig right in to the what, how and why of it all. 

You with me?

Chemical Factors

There are several factors that have to stay in balance in order to keep your water healthy.

The reason for this is that they all work together to keep your sanitizer working effectively.

They include:


Sanitizer is the product you use to keep germs and algae at bay.

Chlorine is the most common, but there are some other options.

For example, if you’d rather not mess with sanitizing chemicals much at all, you can opt for a salt water system.

Or you can opt to use minerals.

But assuming you have a standard hot tub, let’s just cover the basics.

  • Proper Chlorine Levels: 1.0 – 3.0 ppm. Chlorine is the most popular and effective sanitizer you can use for hot tubs or pools, so it is the main one I’ll be talking about most of the time. But if you are opposed to the chemical, there are other options.
  • Proper Bromine Levels: 3.0 – 5.0 ppm. Bromine is another popular choice for hot tubs, especially if they are indoors. It’s not a good choice for outdoor spas or pools, however, because it’s unstabilized and will be quickly eaten up by the sun’s UV rays. The main benefit of bromine is that it’s practically odorless. So, if you really hate the chemical smell of chlorine or if it’s a little harsh for your skin, bromine might be your answer.

It’s important to know what your sanitizer levels are at all times.

If there’s not enough sanitizer, bacteria and algae will start to creep in like uninvited house guests, but if it’s too high, it can actually be pretty harmful to the people in contact with it.

The good news is that it’s not all that hard to fix.

For sanitizer levels that are too low, the fix is as simple as adding more sanitizer.

Don’t go crazy, though! Make sure you read the instructions for your particular product and start adding slowly.

If the levels are too high, that’s a little trickier, but still not crazy difficult. There are a few things you can do to lower them. 

Step 1. The first thing you can do is just wait it out.

Since sanitizer does break down on its own, it will eventually return to normal.

Step 2. If you’re ready to use your hot tub, though, and would rather not wait that long, you can simply dilute the water a bit.

Just grab a big bucket, dip out some water, and replace it with fresh water from the hose. 

Step 3. Once you’ve done this, turn on the jets and allow the water to circulate for about 20 minutes and then retest it.

Step 4. If the levels are extremely high, you may need to dip out quite a bit of water, or drain it completely and start over.


Proper pH Levels: 7.2 – 7.8

Your hot tub’s pH levels will affect how well the sanitizer is working.

What it measures is how acidic or basic the water is: if it’s too low, it’s considered acidic; too high will be considered more basic. 

When pH levels are not properly balanced, your chlorine will not work as efficiently at killing off unwanted bacteria and algae.

But that’s not all.

Water with pH levels that are too low can start to damage your hot tub lining and plumbing, but too high can cause buildup and scaling that clogs things up and is hard to remove. Either extreme can cause skin and eye irritation. 

The problem with pH is how unstable a factor it is.

It’s affected by debris, rain or practically anything else that slips into the water.

So, in order to keep it where it needs to be, you will need to keep some stuff on hand to balance it.

To raise your hot tub’s pH levels, you can use a product containing either sodium carbonate or you just use straight sodium bicarbonate (baking soda).

There are pH increasers you can buy that help take the guesswork out of it by giving you exact amounts and instructions.

Lowering pH levels can be done with muriatic acid or sodium bisulfate.

I recommend getting a product made specifically for this purpose and specifically for hot tubs.

Read the directions and add the amounts appropriate for your spa’s size.

Allow the jets to run and circulate the water for a little while (15-20 minutes should do the trick), then test it again.


Proper Alkalinity Levels: 80 – 120 ppm

Alkalinity essentially measures your water’s ability to neutralize acids.

Basically, alkalinity acts as a stabilizer for your hot tub’s pH levels.

Because pH levels are so unstable, you need your alkalinity to be balanced to keep your pH from bouncing all over the place every time a leaf falls in the water.

This is important because of the damage out of balance pH levels can do to your hot tub and because of the ineffectiveness of chlorine in such an environment.

You raise or lower alkalinity just like you do the pH levels: with a pH increaser (such as baking soda) or decreaser.

Calcium Hardness

Proper Calcium Hardness Levels: 100 – 250 ppm

Calcium hardness is the measure of calcium (or lime) dissolved in your water.

It’s what makes your water feel hard or soft.

If your calcium hardness level is too high, it can cause calcium buildup and scaling that can clog your filter and become difficult to remove. It will also cause your water to look cloudy and dry out your skin when you soak in it.

If it is too low, on the other hand, your water will start looking for places to get more.

In pools, this usually means it starts stealing calcium from the surrounding plaster or cement.

Since hot tubs typically don’t have any of that, it will start to corrode the internal parts of your equipment, like spa heaters and pipes. 

Prevention is the easiest way to keep your calcium hardness levels in check.

I always recommend using a pre-filter when filling your hot tub to filter out most of the junk from your water supply that causes problems.

You can also use a chemical that helps prevent scaling if you already know you have hard water in your area.

Shocking Your Hot Tub

Shocking your water just means giving it a super dose of sanitizer to kill everything bad off and kind of give all your chemicals a reboot.

Shocking your hot tub will keep you from having to drain your water too often, and it’s a good way to make sure you’re not harboring any nasty bacteria that can build up in hot water. 

For chlorine hot tubs, you can use either more chlorine than usual or some non-chlorine shock.

Either way, you’ll need to add the chemicals to the water and let the jets run for a bit to circulate it. Then let it work overnight and test the levels in the morning.

Never let anyone get in the hot tub until the sanitizer levels are back in normal range!

Bromine hot tubs can be shocked with an extra dose of bromine or also with non-chlorine shock.

Bottom Line

Most issues with things like scaling and unbalanced chemicals are usually due to the minerals in the water supply.

That’s why I always recommend starting off right by using your pre-filter to fill your hot tub.

But the great thing about hot tubs is that they’re easier to drain and refill than a pool. So if you have problems keeping something level, you can always start fresh and try again. 

Now you know just about everything there is to know about hot tub chemistry and you are well on your way to being a pro.

But if you run across a problem and need a little advice, we’re always here to help.

What’s your biggest hot tub challenge?

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