Hot Tub pH Keeps Changing? Why It Drifts and How to Fix It

You balance the water, the strip reads perfectly, and a few days later the pH has moved again. Then you correct it, and it moves the other way. The cause is usually simple to find. Here's how to work out why your pH won't settle, and what to do about it.

What pH should a hot tub be?

Keep the pH between 7.2 and 7.6. Outside that range your sanitiser stops working properly, so the water can look clear and still not be clean.

• Low pH (below 7.2) makes the water corrosive. It can sting eyes, and over time it can stain surfaces and attack metal parts.
• High pH (above 7.6) turns the water dull, lets scale form on the shell and heater, and stops your sanitiser dissolving and working as it should.

Alongside pH, check total alkalinity, which should sit at 80 to 120 ppm, and free chlorine, at 1.0 to 1.5 ppm.

First, test properly

A wrong reading can look like drifting pH, so get the test right before you change anything. Our 3-in-1 Test Strips (£5.99 for 50) read chlorine, pH and total alkalinity in one dip. Dip a strip, take it straight out, hold it level for 15 seconds and compare it with the colour chart on the bottle. The strips come in sealed blister packs of 10, so keep the unused ones sealed and dry, because damp strips give odd results.

Why pH keeps changing

The alkalinity is too low

Total alkalinity is the buffer that holds pH steady. When it drops below 80 ppm, the pH has nothing holding it in place, so it swings every time you add a chemical or use the spa. This is the most common reason owners find themselves chasing pH up and down.

The fix is to correct alkalinity first, then pH. If alkalinity is low, add Alkalinity Booster (750g, £10). Add it slowly, in small amounts, because it's easier to add a little more than to bring alkalinity back down. Run the jets for 15 to 30 minutes after each adjustment, then test again.

The water is new and hasn't had its first adjustment

Fresh tap water isn't ready for bathing. Every area's water is different, so after each fill you need to make one initial adjustment to bring alkalinity and then pH into range. After that first adjustment, the water should hold steady.

If you live in a soft water area, expect pH and alkalinity to need a little more attention. In a hard water area, high pH and scale go together, and our guide to hard water and limescale in your hot tub explains how to keep both under control.

It's overcorrected

Adding a big dose of pH Down, finding it's gone too low, then adding pH Up to compensate is a quick way to end up going round in circles. Make small adjustments, let the jets run so the chemical mixes in, and retest before adding any more. The free Canadian Spa Water Test app gives you exact amounts for your spa, so you're not guessing.

The water is full of detergent

If your alkalinity is in range and you're adjusting carefully, but the pH still moves up and down, the water itself is usually the problem. With our chlorine, pH stays stable once you've made that first adjustment after filling. When it starts swinging, it's a sign the water is carrying too much detergent and body products, such as washing powder left in swimwear, shower gel, body lotion and hair products.

No amount of pH Up or pH Down will fix that. The answer is a full water change, which we'll come to below.

How to bring pH back into range

1. Test the water and note pH and total alkalinity.
2. If alkalinity is below 80 ppm, add Alkalinity Booster a little at a time. Run the jets for 15 to 30 minutes and retest.
3. If alkalinity is high, use pH Down. Lowering pH gradually pulls alkalinity down too.
4. Once alkalinity is right, adjust pH. If it's below 7.2, use pH Up (750g, £15). If it's above 7.6, use pH Down (1kg, £15).
5. Run the jets, wait, and retest before adding anything else.
6. When pH and alkalinity are both in range, set your chlorine to 1.0 to 1.5 ppm.

Always follow the dosing instructions on the container. If you want a fuller walk-through of each chemical and the order to add them, our step-by-step hot tub chemistry guide covers it from first fill.

When a water change is the real fix

If pH keeps drifting even with alkalinity in range, change the water rather than adding more chemicals. Most owners change it every 4 to 6 months, depending on how much the spa is used. New owners often find their first change comes sooner, at around 2 to 3 months, while they're using the spa a lot, and then it settles down.

Before you drain, add Spa Flush (£18.99) to the old water, following the label. It breaks through biofilm and the build-up of oils, lotions and cosmetics inside the pipework, so the water pressure can push it out rather than leaving it to contaminate your fresh fill. Badly fouled pipework may need more than one purge. Then drain, clean the filter, refill and make your one initial adjustment.

Our guide to how often to drain and refill a hot tub takes you through the whole process.

How to keep pH steady

• Test two to three times a week, and the day after a busy soak.
• Keep alkalinity at 80 to 120 ppm, so pH has something holding it in place.
• Make small adjustments and retest, rather than adding a large dose in one go.
• Shower before you get in, and rinse swimwear in clean water instead of washing it with detergent or fabric conditioner.
• Keep the water fresh with a change every 4 to 6 months.

Hot tub pH questions

Can I use the hot tub if the pH is out of range?

It's best not to. Outside 7.2 to 7.6 the sanitiser works poorly and the water can irritate skin and eyes. Balance it first, then get in.

If you've tested, balanced and changed the water and your pH still won't settle, call Canadian Spa Company on 01293 824094 and we'll help you work through your readings.