Guide to Testing Swimming Pools and Hot Tubs

Maintaining clean, safe water in swimming pools and hot tubs isn’t just about comfort, it’s critical to ensuring the health and safety of everyone who uses them. Unlike natural water bodies, pool and spa water is artificially treated and re-circulated, making it essential to regularly monitor chemical levels and biological contaminants. Proper testing helps prevent issues such as skin irritation, equipment damage, and even more serious risks like infections from waterborne pathogens, especially in hot tubs, where warmer temperatures can accelerate bacterial growth including E.coli, Pseudomonas and the potentially deadly Legionella bacteria.

This pool guide provides a comprehensive overview of how to test and manage water quality in swimming pools and hot tubs. It covers key chemical parameters such as chlorine, pH, and alkalinity; explains the differences between test strips, liquid kits, and digital testers; and outlines how to conduct and interpret microbiological testing. You’ll also learn best practices for sampling water, identifying common testing errors, and ensuring your testing process is accurate and effective, whether you’re a homeowner or a commercial operator.

Test Your Pool Water

What is swimming pool water?

Swimming pool water is treated water specifically maintained for use in swimming pools, hot tubs and hydrotherapy pools. Impurities introduced into the water from swimmers and the pool environment are managed through mechanical and chemical treatments to ensure the water remains clear, clean, and safe to use.

Pool treatments include the application of chemical disinfectants such as chlorine or bromine, pH balancers, algaecides, and stabilisers. Filtration and circulation equipment is also used to remove debris, while regular testing and maintenance ensure the water remains visually appealing, safe, and comfortable for swimmers.

Why test swimming pool and hot tub waters?

Having a swimming pool or hot tub is a real luxury, but keeping it in tip-top condition all year-round can be quite complex. One of the most important tasks in looking after your swimming pool or spa is the testing regime, which monitors and attempts to balance out the levels of chlorine in the pool with the alkalinity of the water, control the microbiological loading and other factors. Although of course it’s possible to outsource these tasks and get a pool expert in regularly to test for you, it’s something which some owners and leisure operators choose to do for themselves. Most testing doesn’t have to be complex; the key is understanding what you are testing to find, and what to do with the results.

How often should I test my swimming pool?

The three main chemical tests for swimming pool waters are for pH levels, alkalinity and chlorine, and these should be done at least once a week. Checks for other chemicals can be carried out monthly. However, most pool owners prefer to get into the habit of checking their swimming pool water daily during the times of year it is being used.

The main chemical tests for swimming pools are:

pH balance
alkalinity
chlorine

It is also important that regular microbiological testing is carried out to make sure the water treatment programme is working as it should and the pool is safe to use. These tests cannot be done on-site and should be carried out in a specialist lab. We recommend they are done monthly.

Learn more about microbiological testing for pools and hot tubs

Hot tubs and spas use less water and operate at higher temperatures than swimming pools. This can increase the potential for bacterial growth, including Legionella bacteria, Pseudomonas, E.coli and other waterborne pathogens. We therefore recommend that microbiological analysis of hot tubs is also carried out monthly.

If you have an outdoor pool which is only used during the summer months, the full range of chemical water tests should be carried out when you are ready to start using it again as the weather warms up, and again when you want to close it up for the winter.

Every swimming pool is different, and everyone uses their pool in a different way, whether it be a private or commercial pool. It is always best to get advice from the experts first about what tests you should be doing, given the type and location of pool, and the way in which you are using it.

How to use pool testing strips?

By far the most common – and straightforward – way of testing your swimming pool’s water is by using testing strips. These strips are dipped into the water and will change colour depending on the strength of chemicals which are detected. The colour change in the strip can then be compared to a colour chart provided in the instructions. Reading these results from the strips is quick and simple. Another similar, although more complex method, involves adding a testing liquid (called a reagent) to a water sample, or allowing a testing tablet to dissolve in the water, before looking at the colour change in the same way.

Using digital pool testers

The most modern (and most expensive) method of testing is using a digital pool testing kit. These work by submerging electrodes into the water, which detect levels of chemicals and display the results on screen. These kits are expensive though, and are more commonly used by commercial operators such as hotels, spas, leisure centres or sporting venues rather than small home pools. Many swimming pool suppliers offer digital testing kits, and a further option is taking a sample of water and sending if off for independent laboratory analysis rather than doing the tests at home.

Is one water test method better than another?

As long as you are buying your testing equipment from a reliable and reputable supplier, the test strips, tablets, liquid or electronic testing kits should all provide you with an accurate result – assuming of course that you follow the manufacturer’s instructions in full. Most swimming pool owners like the strips because they are so simple and cheap to use. The liquid or tablet options can take longer to produce a result but they might be more accurate too.

Microbiological testing for swimming pools and hot tubs

It’s important that regular microbiological testing is carried out to make sure any treatment of the water is working as it should and the pool is safe to use. These tests are carried out in special labs, not in-field and should target:

E. coli
Pseudomonas aeruginosa
Total coliforms
TVC/Aerobic colony count at 37^oC

Hot tubs and spas should also be tested on a monthly basis for:

E. coli
Pseudomonas aeruginosa
Total coliforms
TVC/Aerobic colony count at 37^oC
Legionella bacteria

If swimmers become ill while using the hot tub or spa more tests may be needed including those for Cryptosporidium, Staphylococcus aureus, Giardia and possibly viruses.

Test Your Pool Water

How to sample your swimming pool water

Swimming pools contain large volumes of water, and this can mean that the chemical composition in one zone of the pool may be different from the water in another area. Evaporation and other environmental factors can mean, for example, that water sampled at the top of the pool is different from water taken from the bottom. In terms of sampling, the best approach is to take a sample from the middle of your pool, preferably from close to the surface.

Understanding the chemical levels in your swimming pool

Although every pool is different, there are some general rules when it comes to pool chemistry which can be applied to pools of all shapes and sizes. Your pool testing kit should also contain information about the optimum levels you are hoping to find through testing.

Chlorine
The optimum level for chlorine in a typical pool is between one and three parts per million (ppm). If the level is a bit too low, then it’s just a case of upping the dose a little. If the levels really are very low, then it might need an initial shock treatment. On the other hand, if you find the levels of chlorine are too high, then just remove the pool-floating dosage tool until the pool returns to the correct levels will do the trick.
pH balance
pH is a measure of the acidity or alkalinity of the water and is a key test. The ideal pH range is anywhere between 7.2 and 7.6, with the optimum measure being 7.4. As long as you have the right chemicals, adjusting the pH level of your pool water is relatively straightforward. There are many different products on the market which are sold as pH increasers and decreasers. pH can be affected by lots of the other chemicals which you add to your swimming pool, so it’s often best to get all the other levels correct first, then finally test the pH balance.
Alkalinity
The level of alkalinity in the water should be anything between 80 and 120 parts per million. Again, adjusting the alkalinity is a simple task if you have the right products. Alkalinity and pH are closely linked, so you may find that once you’ve adjusted alkalinity, you then have to go back and look at pH again.

Common errors when testing pool water

Testing swimming pool water can be a tricky business, and even the experts get it wrong sometimes. Making mistakes or simply getting errors in your testing regime can give you results you weren’t expecting.

What if the test strips don’t change colour?
If you dip a test strip into the water and the strips don’t change colour at all. This is often because the chlorine levels in the pool are too high, and the water simply “bleaches” the strips which stop working. If you suspect this might be the case then dilute your pool sample 50/50 with tap water, and test again. If you now get a result, then too much chlorine in the water is likely to be the issue.
pH test strip shows the wrong colour
Most pH test strips turn a yellow or orange shade to demonstrate acceptable pH levels. If all of a sudden your test strip turns purple instead, then this might also indicate that the test is being affected by too much chlorine in the pool.
Be careful when handling pool test tablets
If you’ve chosen to test your pool water using dissolving tablets, then you must be careful not to touch the tablets as you drop them into the water. Allowing the tablets to come into contact with your skin can make the results inaccurate.
Unnecessary delays when testing
One of the most common reasons for inaccuracy in test results is leaving the water sample for too long before you test it. If you are using a kit which involves you adding liquid to the sample, leaving it too long before returning to read the result can produce inaccurate readings. Always read the manufacturer’s instructions and stick to their suggested timings.
Test kit contamination
When using the tablets or liquids, one common problem is that over time the testing tube itself can become contaminated, which can affect the final results. Never rinse the tubes out with tap water before performing a new test, as this can dilute the sample. Always rinse out the tube with water from the pool, to ensure that all the water has a similar chemical composition.

As every testing kit is different, and every pool is different, it is essential to read the manufacturer’s instructions carefully, and contact their sales support department in the first instance with any queries.

Expert swimming pool water testing solutions

Water Treatment Services offer a comprehensive range of water quality testing and laboratory analysis solutions to support owners and operators of swimming pools, spas and hot tubs. Our experts can provide advice and support to help you identify the most appropriate strategies for the on-going management of your pool facilities.

With offices in London serving the South and South East England, Manchester (North West), Birmingham (Midlands), Bristol (South East England and Wales), Leeds (North and North East) and Glasgow (Scotland), supported by regional teams of specially trained technicians, in-field specialists and consultants we can offer professional, cost effective water management and laboratory analysis solutions across the whole of the UK and internationally.

Contact us today to learn how our expert water management solutions can help keep your pool clean and safe to use.