❄️ Ready to close your pool? CLICK HERE for our new pool winterization video course »

Hot Tub Water Testing: The Complete Guide

By entering your email address you agree to get a weekly email newsletter from Swim University. We'll respect your privacy and unsubscribe at any time.

Is your hot tub water looking a little … off? Maybe a sort of cloudy, or a slightly greenish? Whatever the case, it’s just not the crystal clear water you want to soak in.
Sure hope you have some spa test strips handy, because that’s going to be your first step in diagnosing a problem

The majority of problems with hot tub water come about because it’s not balanced. Before you can add the right chemicals in the right amounts, you have to do some hot tub water testing to determine know what’s going on.

Hot Tub Water Testing is the First Step

Out of all the hot tub maintenance tasks you’ll perform over the life of your hot tub, spa water testing is the one thing you’ll do more often than anything else. At least, it should be.

The only way you’re going to know whether the water contains any contaminants or what the chemical levels are is to test the water. And that means hot tub test strips are your best buddies.

Accurate Spa Water Testing Tips

It’s not as difficult as you may think it is, especially when you have so many testing options available to you:

No matter what testing method you use, the key is to make sure you correctly take the water sample.

How to Collect a Hot Tub Water Sample

What matters just as much as how you collect the sample is that you get it from the middle of your hot tub.

  1. Use a clean cup or bottle (with a cap if you’re taking it to a store).
  2. Hold it upside down so the opening faces the hot tub floor.
  3. Insert the vessel into the water elbow-deep, and then turn it right side up to collect the water.

Important: Do not take the sample near any of the jets. Try to get as close to the middle of the hot tub as possible.

How to Use Spa Test Strips

It may seem counterintuitive, but test strips can sometimes actually be more accurate than liquid test kits because it can be difficult to match up the colors with the chemical drops. They’re also much easier to use.

In addition, hot tub test strips are usually less expensive than liquid testing kits, but you’ll probably end up going through them more quickly, so cost may not really be a factor in which one you choose.

To use spa test strips:

  1. Collect a water sample.
  2. Dip a strip into the water, and quickly pull it out; it doesn’t need to soak.
  3. Hold it still for about 15 seconds. Don’t shake it!
  4. Compare the strip to the guide on the package, matching up the colors on both.
  5. Balance the water accordingly.

You’ll most often use regular hot tub test strips that check for things like pH, alkalinity, and chlorine. But if you also need to test for other components such as iron, copper, and salt, you can find specialty strips.

Our Top Pick
AquaChek Select 7-in-1 Pool and Spa Test Strips
$16.99

Tests for 7 important chemistries in seconds: Total Hardness, Total Chlorine, Total Bromine, Free Chlorine, pH, Total Alkalinity, and Cyanuric Acid.

Buy Now on Amazon Learn More
If you click this link and make a purchase, we earn a commission at no additional cost to you. 11/28/2020 01:48 am GMT

How to Use a Liquid Test Kit

If you go with this option, make sure it includes phenol red and orthotolidine (OTO) to test pH and chlorine, respectively.

  1. Collect a water sample; your test kits should come with a collector.
  2. Add the recommended number of drops of phenol red to the water on the pH side.
  3. Add the recommended number of drops of OTO to the water on the chlorine side.
  4. Hold the collector up in front of a white background, and compare the colors of the water to the color scales on the collector. High pH is indicated by darker shades of red water, and high chlorine is indicated by darker shades of yellow.

You’re shooting for a pH level between 7.4 and 7.6, with 7.5 being ideal.

Important: If the levels are low, the colors will be faint and difficult to see. For better accuracy, be sure to hold the collector up against a white background. Just a sheet of paper will do.

5-Way Test Kit with Case
$35.59
Buy Now Learn More
If you click this link and make a purchase, we earn a commission at no additional cost to you. 11/28/2020 06:49 am GMT

Professional Water Testing

To take a water sample to a dealership for testing, collect the sample the same way you would if you were going to perform spa water testing at home. They may take walk-ins, but call ahead to make sure they’ll be able to help you.

When to Test Hot Tub Water

To make sure the water stays sanitized and that alkalinity and pH are always in balance, perform hot tub water testing at least once a week. If your spa gets a lot of use, it’s been rained on, test it more often. And of course, test it whenever you add chemicals.

We highly recommend you take a sample to a store once a month for a professional test, as well as when you open and close your hot tub.

If your hot tub isn’t properly balanced, you can follow our guides about balancing pH in a hot tub and how to lower alkalinity in a hot tub.

Keep Contaminants at Bay

If your water comes from a well, you may have higher levels of iron and copper in your hot tub. If you use city water, it may be hard, which means it contains higher levels of calcium and magnesium.

All of those factors can cause problems in your spa, including chemicals imbalances, hot tub stains, and other surface damage, meaning you’ll need to test the water more often.

Prevention costs a lot less than repairs.

Learn How to Keep Your Hot Tub Clean and Clear All Year

You'll save $100 right away with this easy-to-follow digital ebook and video course. This is the guide that hot tub manufacturer doesn’t provide you.

Click Here to Learn More
The Hot Tub Handbook

How to Test for Copper

If you have greenish or black stains on the shell or cloudy spa water, you may have a copper problem. It builds up in two ways.

First, the water you fill your hot tub with may contain copper. As the water evaporates from your spa, copper is left behind behind. As this continues, copper levels rise, leading to cloudy water and stains.

Because the spa has lost water to evaporation, you’ll add more, which means you add more copper, and then more water evaporates, and that leads to … yup, more copper in your hot tub.

Second, copper elements in your hot tub filter or plumbing may corrode due to a low pH level, and that corrosion will end up in the water. The longer you let it go, the worse it will get.

How to Test Your Hot Tub Water for Copper

Most spa test strips don’t test for metals. In addition to the ones you get for pH, chlorine, and other standard factors, you’ll need a set made especially for metal testing.

Metals Test Strips
$19.84
Buy Now Learn More
If you click this link and make a purchase, we earn a commission at no additional cost to you. 11/28/2020 08:49 am GMT

How Often to Test for Copper

It’s essential to test the pH level weekly as that can lead to a copper issue. But as for copper itself, you should only have to test the water once a month.

How to Fix High Copper Levels

After testing your hot tub water, if you do find it contains a high level of copper, you have a couple of options to remedy it.

Metal Sequestrant

Usually a liquid you add to your hot tub water, a sequestrant binds with metals—specifically, iron, copper, and nickel—in the water to prevent them from oxidizing, or rusting, which is what causes stains. Once the metal particles are bound, they can be removed by the hot tub filter.

Be aware that a sequestrant does not remove metals from the water. And sequestrant breaks down over time, so you may need to add it on a regular basis if you have high metal levels in your water.

Learn what chemicals and minerals are present in the water where you live, and then follow the manufacturer’s instructions to ensure you use the sequestrant properly.

Hot Tub Things Metal Out - 16 oz.
Buy Now Learn More
If you click this link and make a purchase, we earn a commission at no additional cost to you.

Hose Filter

If your water source has high metal levels, a hose filter can keep at least some of them out when you fill your hot tub.

Pre Fresh All-Purpose Hose Filter
$29.95
Buy Now Learn More
If you click this link and make a purchase, we earn a commission at no additional cost to you. 11/28/2020 01:47 am GMT

Test for Iron

Another good reason to test the pH level weekly is that a low level can also cause iron levels to rise, if there’s iron present in the source water.

Again, if your water comes from a well, it may have high metal levels. The process of iron being deposited in the spa is the same as for copper. Only in this case, the water will be brown or rust-colored. Ew.

How to Test Your Hot Tub Water for Iron

Usually, the same strips or kit that test for copper will also test for iron. It just makes sense as both are metal elements.

The only difference will be that once you’ve filled the test tube with a sample of your hot tub water, you’ll add a special solution made to test for iron. Then compare the water color to the color coded chart included with the kit.

Metals Test Strips
$19.84
Buy Now Learn More
If you click this link and make a purchase, we earn a commission at no additional cost to you. 11/28/2020 08:49 am GMT

How Often to Test for Iron

Once a month. But remember, test the pH level more often than that to keep iron from staining the spa shell.

How to Fix High Iron Levels

Again, because they’re both metal elements, you can fix an iron problem with the same methods used for copper: metal sequestrant, keep the pH level steady, and use a hose filter when filling your spa.

Test for Calcium

Depending on where you live, you may have to deal with hard water, which means your water source has high levels of calcium and magnesium. If you see evidence of it on your shower head, you probably also see hot tub scale from time to time.

You may also have the opposite problem—too little calcium in the water. This can cause corrosion on fixtures, surfaces, and equipment. Left too long, it can get to the point where repair isn’t possible, only expensive replacement.

The solution is to keep the calcium hardness level balanced.

How to Test Your Hot Tub Water for Calcium

For total hardness—the measurement of both calcium and magnesium salts—you’re better off using a professional grade hot tub water testing test kit rather than spa test strips.

Shoot for between 175 parts per million (ppm) and 250 ppm.

Taylor Technologies Complete Test Kit
$63.99
A simple but effective liquid test kit for chlorine pools and hot tubs.
Buy Now Learn More
If you click this link and make a purchase, we earn a commission at no additional cost to you. 11/28/2020 05:47 am GMT

How Often to Test for Calcium

Once you see scale forming, it’s too late for prevention and you’ll have to move into remedy mode. To keep that from happening, test for calcium hardness at least once a month.

But if your water source is either too soft or too hard, it’s a good idea to test more often, about once every two weeks. If your water has a serious calcium hardness issue, test once a week. You’ll be glad you did.

How to Fix High Calcium Levels

You have two options here.

Replace Some of the Water

If your water source isn’t hard, you can drain a couple of inches of water from your hot tub and replace it with softer water.

Use a Descaler

Follow the manufacturer’s instructions to ensure you use the proper amount.

How to Fix Low Calcium Levels

Easy. Just use a calcium hardness increaser. Again, follow the manufacturer’s instructions.

Test for Salt

If you have a salt water hot tub, the salt you add to your hot tub is converted to chlorine. But you still need to make sure you don’t have too much or too little salt in the water. Too much can cause the chlorinator to work too hard, while too little might mean the water isn’t properly chlorinated.

How to Test Your Hot Tub Water for Salt

To go easy on the chlorinator and keep the water sanitized, you want the salt level to be between 2,000 ppm and 3,000 ppm, with 2,500 ppm being ideal. You’ll need specialty hot tub test strips to measure salt levels. You use these the same way you use standard spa test strips.

How Often to Test for Salt

You can get away with testing the salt level just once a month because it’s not as difficult to balance as other factors like pH. But if your hot tub is outside and uncovered, make sure to test after it rains, or after the spa has seen a lot of use.

How to Fix a High Salt Level

Once salt dissolves into water, it stays there, so the only way to reduce a too-high salt level is to drain a little water from the spa, then add fresh water.

How to Fix a Low Salt Level

Ooh, this one’s a toughie. Yeah, it’s gonna take a while and a lot of work. You ready? You need to … add more salt. Yup, that’s it. Fixed.

After you add salt, give it some time to dissipate, then break out the hot tub test strips again. Just add a little at a time so you don’t accidentally raise the level too much.

Always Be Testing

This is truly the easiest way to head off problems in your hot tub. Keep a supply of spa test strips on hand, and you’ll always know the status of the water you soak in, and you can address issues before they become headaches.

Happy Soaking!

Matt Giovanisci is the founder of Swim University® and has been in the pool and spa industry since 1995. Since then, his mission is to make pool and hot tub care easy for everyone. And each year, he continues to help more people with water chemistry, cleaning, and troubleshooting.

Learn How to Keep Your Hot Tub Clean and Clear All Year

You'll save $100 right away with this easy-to-follow digital ebook and video course. This is the guide that hot tub manufacturer doesn’t provide you.

Click Here to Learn More
The Hot Tub Handbook

Subscribe To Our FREE Hot Tub Owner Newsletter

By entering your email address you agree to get a weekly email newsletter from Swim University. We'll respect your privacy and unsubscribe at any time.