How to raise pH in a hot tub

Here is how to raise pH in hot tub water: test first, and if total alkalinity is below 80 ppm, raise that before anything else. Then add pH increaser (sodium carbonate, also called soda ash) in small doses with the jets running, wait, and retest until pH sits between 7.2 and 7.6.

Low pH is one of the most common water problems, especially in soft water areas. The good news is that it is easy to correct once you understand why it happens. This guide covers the right order of steps, how to avoid overshooting, a gentle method that uses only air, and how to stop pH falling again.

Why does hot tub pH matter?

pH measures how acidic or alkaline your water is, on a scale from 0 to 14. Seven is neutral. Below 7 is acidic and above 7 is alkaline. Hot tub water works best when it is very slightly alkaline.

The widely accepted target is 7.2 to 7.6. Some manufacturers allow up to 7.8, so check your owner's manual and follow it if it differs. Our guide to hot tub pH levels and the range to aim for explains the scale in more detail.

Within that range, aim for the middle, about 7.4. pH drifts a little between tests, and starting from the middle means a small drift in either direction still leaves the water in range.

Once pH is right, your sanitiser should also sit in its target range. For a domestic hot tub that usually means about 3 to 5 ppm free chlorine. Bromine targets vary between sources, with 3 to 5 ppm and 4 to 6 ppm both commonly given, so follow the figure in your manual.

pH also controls how hard your chlorine works. Free chlorine exists in two forms: hypochlorous acid, which kills germs quickly, and hypochlorite, which is far weaker. At about pH 7.5, roughly half of the free chlorine is hypochlorous acid. At pH 8.0, only about a quarter or less is.

This means a normal-looking chlorine reading kills germs far more slowly when pH is high, because the test counts both forms. It is one more reason to raise pH in small steps and avoid overshooting above the target range.

When pH drops below 7.2, the water becomes more acidic and starts to cause problems. Some are uncomfortable for bathers, and some cost money to repair. Watch for these signs:

• Stinging or red eyes and itchy, dry skin after a soak
• A stinging or irritated nose during or after a soak
• A strong chlorine smell even when the sanitiser reading looks normal
• Sanitiser that seems to disappear faster than usual
• Corrosion or pitting on metal parts such as the heater element, jet trims and fittings
• Worn or damaged pump seals, which can lead to leaks
• Etching or dulling of the shell surface over time
• Green or blue staining from dissolved metals in some cases

Corrosion damage builds slowly and is not always visible until a part fails. Heater elements are expensive to replace, so keeping pH in range is cheaper than repairing the damage that low pH can cause.

What causes low pH in a hot tub?

pH rarely drops for no reason. Knowing the cause helps you fix it once rather than chasing the same problem every week. These are the usual reasons:

• Soft fill water: water in many parts of Scotland, Wales, the north and west of England and much of Scandinavia is naturally soft and low in alkalinity, so pH has nothing holding it steady
• Topping up with soft water: replacing water lost to evaporation or splash-out with fresh soft water dilutes alkalinity and can pull pH down
• Chlorine granules: dichlor, the most common hot tub chlorine, is mildly acidic and nudges pH down a little with every dose
• Chlorine tablets: trichlor tablets are strongly acidic and are generally not recommended for hot tubs. They dissolve to a pH of about 2.8 to 3, so tubs dosed with them tend to suffer constant low pH, and switching to dichlor granules or bromine often ends the problem
• Bromine tablets: most bromine tablets are slightly acidic and can lower pH gradually
• Non-chlorine shock: monopersulphate (MPS) shock lowers both pH and alkalinity over time. It is strongly acidic, with a solution pH of about 2, so an overdose of shock can drop pH noticeably in a single treatment
• Bathers: sweat, body oils and other residues add acidic compounds to the water
• Overdosing pH reducer: adding too much at once, or adding more before the first dose has mixed
• Rainwater getting in when the lid is left off, as rain is usually slightly acidic

It helps to know that hot tub pH naturally tends to creep upward. Heat and jet use drive carbon dioxide out of the water, and losing carbon dioxide raises pH. So a tub whose pH keeps falling points to a specific cause, most often soft fill water or an acidic sanitiser.

If your tub drops low every few days, the fill water and your sanitiser are the first two things to look at. Both can be managed, as covered later in this guide.

Top-ups matter as well as full refills. After any significant top-up, retest alkalinity and pH, because the fresh water may have diluted the buffer enough to let pH start falling.

What do you need before you start?

Gather everything first so you are not leaving chemicals open while you look for a scoop. You will need:

• Fresh test strips, a liquid test kit or a digital water tester that reads pH and total alkalinity, checked for an in-date expiry
• Alkalinity increaser (sodium bicarbonate), in case alkalinity is low
• pH increaser (sodium carbonate, sold as pH plus, pH up or soda ash)
• A clean plastic bucket kept only for hot tub chemicals, because both alkalinity increaser and pH increaser are pre-dissolved in tub water before they go in
• A clean, dry measuring scoop or kitchen scales kept only for hot tub chemicals
• Chemical-resistant gloves and eye protection
• Your hot tub's water volume in litres, from the manual or the manufacturer's website

You also need time. Set aside around six to eight hours for a full cycle of testing, adjusting and confirming. Alkalinity needs several hours to settle before you retest it, and pH needs its own circulation and retest time after that. Start in the morning rather than shortly before you plan to use the tub.

Gloves and eye protection matter because pH increaser is a fine, strongly alkaline powder. Dust can lift into the air as you scoop or pour it, and it can irritate eyes and skin. Scoop slowly, keep your face away from the container, and if any gets in your eyes, rinse with plenty of clean water and follow the first aid advice on the label.

Strips are quick and simple, liquid kits are more accurate, and a digital tester gives the most precise readings. If you use strips, choose multi-pad strips that also read sanitiser, not only pH and alkalinity. That way you can check all three steps of the balancing order with one strip.

Store test strips in a cool, dry place, not a steamy bathroom, as moisture spoils the pads. Use an opened pot within about three months, and replace your stock at least once a year.

Knowing your volume matters because product doses are given per 1,000 litres or similar. Most hot tubs hold somewhere between 800 and 1,600 litres (roughly 210 to 420 US gallons), and a guess can easily lead to a double dose.

How to test your hot tub water

Every dose decision depends on your test result, so a careful sample is worth the extra minute. A reading taken from still surface water can be quite different from the water in the rest of the tub.

1. Remove the lid and run the jets for two or three minutes, so the reading reflects the whole tub rather than a still surface layer.
2. Switch the jets off so the water settles enough to sample.
3. Take the sample from roughly elbow depth, about 30 to 45 cm (12 to 18 inches) below the surface, not from the top few centimetres, where readings can be skewed.
4. With test strips, take one strip out with dry fingers and close the container straight away. Wet fingers in the pot can spoil the remaining strips.
5. Dip the strip for about two seconds, then hold it flat. Do not shake off the water.
6. Wait for the time stated on the pack, usually 15 to 30 seconds, then compare the colours with the chart.
7. Compare colours in natural daylight where you can, as artificial light can make shades look different.
8. Write down every reading before adding any chemical.

Why should you fix alkalinity before pH?

This is the step most people skip, and it is the main reason pH keeps going wrong. Alkalinity and pH are linked, and alkalinity controls how stable pH is.

What is total alkalinity?

Total alkalinity measures the dissolved alkaline substances in your water, mainly bicarbonates. These act as a buffer. They absorb acids before those acids can push the pH down.

Most of that alkalinity is bicarbonate, with smaller amounts of carbonate and hydroxide. The buffer works in both directions, absorbing alkaline additions as well as acidic ones. So when alkalinity is in range, it also stops pH jumping too high after a dose of pH increaser.

The usual target is 80 to 120 ppm. Below 80 ppm, the buffer is too weak. pH then swings wildly with every chemical dose or busy evening in the tub, often called pH bounce. If you raise pH without fixing low alkalinity, the pH will usually fall back within a day or two.

How to raise alkalinity in your hot tub

Use an alkalinity increaser, which is sodium bicarbonate. Follow the label dose for your volume, add it with the circulation running, then wait and retest before adding more. Our step-by-step guide on how to raise alkalinity in a hot tub walks through the dosing.

Pre-dissolve the measured dose in a bucket of water taken from the tub before you add it. Pour the solution slowly around the edge of the tub with the jets on high, then run them for 15 to 30 minutes. This avoids a concentrated layer sitting on the surface and cloudiness from undissolved powder.

As a rough check on a label dose, about 18 g (0.6 oz) of sodium bicarbonate per 1,000 litres raises total alkalinity by about 10 ppm. For example, raising a 1,500-litre tub from 60 to 80 ppm needs roughly 55 g. Products vary, so treat this as a sense check and follow the label.

When alkalinity needs a big rise, split the total into two or more doses a few hours apart and retest between them. Adding it all at once makes overshooting more likely.

Alkalinity takes time to settle. After each dose, wait around four to six hours with the circulation running before you retest alkalinity.

Raising alkalinity also lifts pH a little. Once alkalinity reads 80 ppm or more, retest pH before reaching for pH increaser. Quite often, the pH will already be close to the target range and needs only a small top-up.

Always correct total alkalinity first, then pH, then sanitiser. Working in this order saves chemicals and stops pH from crashing again a day later.

How to raise pH in a hot tub with pH increaser

Once alkalinity is in range, pH increaser is the quickest way to lift a low reading. It contains sodium carbonate, which is strongly alkaline, so a small amount makes a big difference in a small volume of water.

1. Switch the jets and circulation pump on so the water is moving. Remove the lid and leave it off while you work.
2. Test pH, alkalinity and sanitiser and write down all three readings, so you have a baseline to compare against later.
3. Put on gloves and eye protection.
4. Check the product label for the dose per 1,000 litres and work out the amount for your tub's volume.
5. Measure out about half of that amount. Starting low protects you from overshooting.
6. Unless the label says otherwise, pre-dissolve the granules in about 4 litres (about 1 US gallon) of water drawn from the tub in a clean bucket. Stir for 30 to 60 seconds until no granules remain, because undissolved granules cause white cloudiness and concentrated pockets that give false high readings. Always add the chemical to the water, never water to the chemical.
7. Pour the solution slowly across the water near the jets, where circulation is strongest.
8. Run the jets for at least 20 to 30 minutes, or for the time given on the label.
9. Retest pH, taking the sample away from where you poured the chemical, as water near the addition point reads high until it is fully mixed. If pH is still below 7.2, repeat with another small dose and wait again.
10. Retest again a few hours later to confirm pH has stabilised before deciding on any further dose.
11. Once pH sits between 7.2 and 7.6, check your sanitiser level and adjust it if needed.

How much pH increaser should you add?

There is no single amount that suits every tub. Products vary in strength, and the right dose depends on your volume, how low the pH is and your alkalinity. The product label is the most reliable guide, and you should follow it over any general advice.

As a rough check on a label dose, about 4 to 5 g of sodium carbonate per 1,000 litres raises pH by roughly 0.2 when alkalinity is in range. That means a 1,500-litre tub needs only about 6 to 8 g per step.

The table below scales that figure for common tub volumes. It assumes alkalinity is already between 80 and 120 ppm. Use it as a sense check against your product label, not a replacement for it.

Even for a larger rise, split the total into smaller doses rather than adding it all at once. Add the first part, circulate, retest, and only then add the next part.

A cautious approach is to add half the label dose, wait, then retest. Hot tubs hold far less water than a pool, so even a small extra scoop can push pH well above 7.8. Bringing pH back down then means adding pH reducer, which costs you more chemicals and time.

Limit yourself to two doses of pH increaser per day, however low the reading. If pH is still low after the second dose, wait until the next day and retest before adding more.

Never mix pH increaser with any other chemical, either dry or in a bucket. Add each product to the tub separately, let it circulate, and store chemicals sealed, dry and out of reach of children.

Can aeration raise pH without chemicals?

Yes. Aeration is a slow, gentle way to lift pH, and it is ideal when pH is only slightly low or when alkalinity is already high. It uses no chemicals at all.

Water holds dissolved carbon dioxide, which forms a weak acid and keeps pH lower. When you churn the surface with air, carbon dioxide escapes into the air. As it leaves, pH rises, while total alkalinity stays roughly the same.

1. Test and note your pH and alkalinity.
2. Remove the lid completely so carbon dioxide can escape freely.
3. Turn the jets on to full and open all the air controls, sometimes called venturi valves, so air is drawn into the jets.
4. Switch on the air blower if your tub has one.
5. Switch on any waterfall or cascade features, as they add surface movement that helps drive off carbon dioxide.
6. Retest pH every 30 to 60 minutes. Put the lid back on once pH reaches about 7.4 to 7.6, because pH keeps rising for as long as aeration continues.
7. Repeat over several sessions if needed. It can take a few hours or more than one day to see a full change.
8. Put the lid back on between sessions, so the water does not lose too much heat.

Do not leave a tub aerating unattended for many hours. As a firm limit, never leave the lid off and the jets running for more than about four hours without checking pH, because it can climb past 8.0. High pH leads to scale on the shell, jets and heater.

If your hot tub is indoors, open windows or doors while you aerate. Running the jets with the lid off also lets heat escape, so expect your heater to work harder for a while afterwards.

Baking soda or soda ash: which should you use?

These two are often confused because the names sound alike. Both are sodium compounds, but they do different jobs. Baking soda is sodium bicarbonate, the active ingredient in most alkalinity increasers. Soda ash is sodium carbonate, the active ingredient in most pH increasers.

Baking soda can raise pH, but only slowly and by a limited amount. It is the better choice when both pH and alkalinity are low. Our article on using baking soda to raise hot tub alkalinity explains when the kitchen product is suitable and when to choose a dedicated spa product.

Why does hot tub pH keep dropping or refuse to move?

If you have corrected pH and it will not stay put, one of these situations is usually behind it. Each has a clear fix. Before you act on any surprising reading, confirm it with a second test or a liquid kit, so you do not dose based on a faulty result.

pH drops again within a day or two

This almost always means total alkalinity is too low. Retest alkalinity and bring it up to at least 80 ppm. If you are in a soft water area, expect to top up alkalinity after every refill and check it weekly.

When you rebuild alkalinity, aim for the middle of the range, about 90 to 100 ppm, rather than stopping just above 80 ppm. Get there with several small doses spaced a few hours apart, retesting between them.

Alkalinity is high but pH is low

This usually happens when too much alkalinity increaser has been added, or when the fill water is naturally high in alkalinity but low in pH.

Adding pH increaser here would push alkalinity even higher. Use aeration instead, as it raises pH without changing alkalinity much. If alkalinity is well above 120 ppm, lower it slowly with pH reducer first, then aerate to bring pH back up.

pH swings high then low

Large doses and low alkalinity together cause this. Raise alkalinity into range, then use smaller doses of pH chemicals and allow full circulation time before retesting. If you have overshot and need to come back down, see our guide on how to lower pH in a hot tub.

Water turns cloudy after adding pH increaser

A brief haze can appear when pH rises quickly, especially if calcium hardness is high. Calcium can come out of solution as tiny particles. Keep the filter running, retest after a few hours and check calcium hardness against the usual target of 150 to 250 ppm. If the haze stays, clean or rinse the filter.

Cloudiness can also mean the sanitiser level is too low to deal with organic matter from bathers. Check sanitiser as well as calcium, and bring it back into range if it has dropped.

If every reading is in range and the water still stays cloudy, clean the filter or check it for clogging first. After that, a hot tub clarifier is an option, used at the dose on the label.

Calcium hardness that is too high or too low

Calcium hardness above about 250 ppm leads to scale on the shell, jets and heater. The only practical way to lower it is to partially drain the tub and dilute the water with fresh water.

Calcium advice for soft water areas differs between manufacturers. Some recommend adding calcium increaser, and others advise against it for their models. Check your manual before adding calcium.

Where your manufacturer approves raising calcium, use calcium hardness increaser, which is calcium chloride. Very soft water can be aggressive towards surfaces and fittings, which is why some manufacturers recommend it.

pH will not change whatever you add

First, check your test strips or reagents. Out-of-date or damp strips give false readings, so try a fresh pack or ask a local pool and spa shop to test a sample. Next, confirm the circulation pump is actually running when you dose.

If readings are accurate and pH still behaves strangely, the water may be old and loaded with dissolved solids from months of chemicals and bathers. At that point, draining and refilling is the most reliable fix.

You can measure total dissolved solids (TDS) with an inexpensive digital TDS meter. Test your fill water once to get a baseline. When the tub reads about 1,500 ppm above that baseline, it is time to change the water.

Common mistakes, and when to drain and refill

Most pH problems come from a few avoidable habits. Avoid these:

• Adjusting pH before checking alkalinity
• Adding a full dose at once instead of half and retesting
• Testing straight after dosing, before the chemical has mixed
• Dosing with the pump switched off
• Adding any chemical while people are in the tub
• Guessing the water volume
• Using pool chemicals without checking the strength and dose for a small volume
• Using trichlor tablets meant for pools, which are strongly acidic
• Adding several chemicals at the same time

Pool products often contain the same active ingredient as spa products, but the doses are written for tens of thousands of litres. If you use one, check the active ingredient and work the dose down carefully for your volume.

A hot tub also behaves differently from a pool. Its high temperature, around 37 to 40°C (99 to 104°F), speeds up chemical reactions and sanitiser loss. With so little water per bather, each person affects the chemistry far more than in a pool, which is why small doses and frequent testing matter.

Draining and refilling is the better choice when chemical corrections no longer hold, the water looks dull or foamy despite balanced readings, or the water has been in use for several months. Many manufacturers suggest a full change every three to four months, but the right interval depends on use, so follow your manual.

A partial drain is an alternative when dissolved solids or calcium are high but the tub is not yet due a full change. Drain part of the water, top up with fresh water, then retest and rebalance in the usual order.

Clean water also matters for hygiene. The UK Health and Safety Executive publishes guidance on controlling Legionella bacteria in water systems, including spa pools, and the Pool Water Treatment Advisory Group sets out industry standards for pool and spa water treatment.

Summary: raising pH the right way

Test first, and fix total alkalinity before you touch pH. Then add pH increaser in small doses with the jets running, wait at least 20 to 30 minutes and retest until pH reads 7.2 to 7.6. For a small lift, or when alkalinity is already high, aerate with the lid off and the air controls open.

To stop pH falling again, keep alkalinity at 80 to 120 ppm, be aware that soft water and chlorine granules slowly pull pH down, and test two or three times a week. For the full picture of how each reading affects the others, read our guide to hot tub water balance: pH, alkalinity and sanitiser in the right order.

Frequently asked questions

How long after adding pH increaser can I use the hot tub?

Most products advise running the circulation for at least 20 to 30 minutes, then retesting. Only get in once pH, alkalinity and sanitiser are all back within range. If the label gives a different waiting time, follow the label. If you added several chemicals in one session, allow each one to circulate fully and retest before anyone uses the tub.

Does shock raise or lower pH in a hot tub?

It depends on the type. Non-chlorine shock (monopersulphate) tends to lower pH and alkalinity. Dichlor shock is mildly acidic, so it nudges pH down slightly. Calcium hypochlorite shock raises pH, but many hot tub manufacturers advise against it because it adds calcium. Lithium hypochlorite shock is also alkaline and raises pH in the same way. Check pH a few hours after shocking and correct it if needed.

Can I raise pH in my hot tub quickly?

pH increaser is the fastest method, and you will usually see a change within 30 minutes of circulation. Resist the urge to add a large dose to speed things up. Overshooting is common in small volumes, and correcting high pH takes longer than adding a second small dose. Aeration works too, but it is much slower.

Why is my pH low after refilling the hot tub?

Fresh water in soft water areas is often low in alkalinity, so pH has little buffering and drifts down quickly. Test the new water before adding sanitiser, raise alkalinity to 80 to 120 ppm, then adjust pH. Your water supplier can tell you how hard or soft your local supply is.

Is low pH or high pH worse for a hot tub?

Both cause problems, just different ones. Low pH is corrosive and can damage the heater, metal fittings and shell, and it irritates eyes and skin. High pH encourages scale and cloudy water and makes chlorine work less well. Low pH tends to cause damage faster, so correct it promptly when you see it.

How often should I test pH in a hot tub?

Testing two or three times a week suits most private hot tubs, and more often after heavy use, after shocking or after a refill. Test before you get in, not straight after adding chemicals. Keep a simple log of readings, as patterns help you spot whether soft water, your sanitiser or bather load is pulling pH down.