Muriatic Acid for Hot Tubs and Pools: Safe Use, UK Rules and Alternatives

Last updated: September 2026

Muriatic acid is one of those chemicals that every pool owner in the USA seems to have in the shed, and that UK hot tub owners have often never heard of. It’s hydrochloric acid, and it’s very good at one job: bringing down pH and total alkalinity. It’s also properly dangerous if you handle it carelessly, and in the UK the rules on buying it changed in 2023.

This guide covers what it is, when to use it on a hot tub or pool, how to add it safely, the UK and US differences, and the gentler alternatives most hot tub owners are better off with.

What is muriatic acid?

Muriatic acid is a solution of hydrogen chloride in water: hydrochloric acid. “Muriatic” is the old name, and it’s still what the pool trade calls it in the USA. Pool-grade products there are typically around 31% HCl. For example, HASA’s muriatic acid safety data sheet lists 31.45% HCl, with the warning “causes severe skin burns and eye damage” and “may cause respiratory irritation”. You’ll also see reduced-strength and “low-fume” versions, which are a bit easier to live with.

UK and US names

What it is	Common names	Where you’ll see it
Hydrochloric acid (HCl), liquid	Muriatic acid (USA), hydrochloric acid, spirits of salts (UK)	US pool shops and hardware stores. In the UK, mainly brick and patio cleaners
Sodium bisulfate, granules	pH minus, pH decreaser, pH down, dry acid	The standard hot tub pH reducer on both sides of the Atlantic
Dilute sulphuric acid, liquid	Liquid pH minus	UK and EU pool shops. For example, Bayrol pH Minus Liquid Domestic is 14.9% sulphuric acid

UK rules: you may need a licence

This is the big UK/US difference. Under the Control of Explosives Precursors and Poisons Regulations, hydrochloric acid above 10% w/w is a regulated explosives precursor. The Home Office’s guidance for home users says members of the public must provide a valid EPP licence and photo ID to buy regulated substances above the concentration limit, with the offences in force from 1 October 2023. Sulphuric acid is regulated above 15%, which is why domestic liquid pH minus products sit just under it. Trading Standards guidance also notes that corrosive products can’t be sold to under-18s or delivered to a residential address.

In practice, UK hot tub owners don’t need hydrochloric acid at all. Use granular pH minus (sodium bisulfate) or a domestic-strength liquid pH minus. In the USA, muriatic acid is sold freely, but the handling rules below apply just the same.

Why pH and alkalinity matter

pH measures how acidic or alkaline the water is, on a scale of 0–14, where 7 is neutral. In a hot tub, pH tends to creep up on its own, because heating and the jets drive carbon dioxide out of the water. So you’ll use a pH reducer far more often than a pH increaser.

pH too low (under 7.2)	pH too high (over 7.6)
Corrodes heater elements, pump seals and metal fittings	Chlorine works much less well
Etches grout, render and plaster	Scale on surfaces, the heater and jets
Stinging eyes and itchy skin	Cloudy water and eye and skin irritation

Total alkalinity (TA) is the buffer that holds pH steady. Fix TA first (80–120 ppm), then pH (7.2–7.6). There’s a full explanation, including the order to adjust things in, in my hot tub chemical levels guide.

How to lower pH or alkalinity with muriatic acid

These steps are for a hot tub. For a pool, the principle is the same, with bigger doses.

1. Test first. Test TA and pH, and write the numbers down. If TA is high, that’s what you’re correcting first. If only pH is high, you’re after a smaller dose.
2. Work out a small dose. Use the product label or your test kit maker’s dosing chart for your water volume. A hot tub holds a tiny fraction of a pool’s water, so doses are measured in millilitres or fluid ounces, not cups. Start with about half of what the chart says. You can always add more.
3. Get your kit on (see the safety section below) and work outdoors, upwind.
4. Dilute it. Fill a clean plastic bucket with tub water, then add the acid to the water, slowly. Never the other way round.
5. Add it to the tub slowly with the circulation or jets running and the lid off. Pour it in front of a return jet, away from the skimmer and filter, and never over the heater or directly on to tiles or grout.
6. Wait and retest. Give it at least 20–30 minutes of circulation (or whatever the label says), then retest pH and TA before adding any more.
7. To lower TA without dragging pH down too far, run the jets and air blower with the lid off after dosing. Aeration drives off carbon dioxide and brings pH back up without adding alkalinity. It may take a couple of rounds.

You don’t need to drain or flush the tub afterwards. Once the acid has mixed in, it’s done its job. Just keep the lid off until you’ve retested and pH is back in range before anyone gets in.

How to handle muriatic acid safely

This is the section to read twice. Hydrochloric acid burns skin and eyes, and the fumes attack your lungs and anything metal nearby.

Protective kit

• Chemical splash goggles (not just glasses) or goggles plus a face shield.
• Chemical-resistant gloves (rubber or nitrile), long sleeves and closed shoes.
• Ventilation. The SDS says to use it only outdoors or in a well-ventilated area. Don’t lean over the bottle when you open it.

Golden rules

• Always add acid to water. Pouring water into concentrated acid can make it spit.
• Never mix acid with chlorine or anything else. Acid plus chlorine (granules, tablets, liquid or household bleach) releases chlorine gas. The HASA SDS says not to mix it with chlorine-type bleaches or other household chemicals, and the CDC’s pool chemical safety guidance says don’t mix pool chemicals, and store incompatible chemicals away from each other.
• Use a dedicated plastic bucket and jug, rinse them after use, and never use them for chlorine.
• Keep kids and pets well away while you work.

Storage

• Keep it in its original container, tightly closed, upright and locked away.
• Store it away from chlorine, and ideally in a different building or cupboard altogether.
• Keep it cool. The CDC says to store pool chemicals below 35°C (95°F).
• Don’t store it in your control room. Acid fumes creep out of even closed bottles and corrode metal: spa pack terminals, pump motors, heater connections and anything steel. The SDS says to keep it away from metals, so keep chemicals well away from your electrics. There’s more in control room design considerations.

If it goes wrong

• Skin: rinse immediately with plenty of water for 15–20 minutes and remove contaminated clothing.
• Eyes: rinse with running water for at least 15 minutes and get medical help straight away.
• Breathed in fumes: get into fresh air, and get medical help if you have any breathing problems.
• Spills: the SDS recommends neutralising with soda ash (sodium carbonate) or lime with plenty of water.
• Disposal: don’t pour neat acid down a drain or on the garden. Follow the label and your local council or waste authority’s rules.

Muriatic acid or pH minus for a hot tub?

	Muriatic (hydrochloric) acid	pH minus (sodium bisulfate)
Form	Liquid, fuming	Granules
Handling	Corrosive fumes and splashes	Easier. Still an acid, so wear gloves and goggles
Side effects	Adds a little chloride	Adds sulfate, which builds up until you change the water
UK availability	Licence needed above 10%	Widely sold for hot tubs
Best for	US pools, big TA reductions	Most hot tubs

For most hot tub owners, I’d use pH minus. The doses are small and easy to measure, and it’s far easier to store safely. Muriatic acid makes more sense on a swimming pool, or if you’re a US owner who’s comfortable handling it and wants to knock back high alkalinity. You change the water in a hot tub every few months anyway, so sulfate build-up from pH minus isn’t a big issue.

Other uses around the tub or pool

Acid has a few other uses, but be careful. Most of the surfaces around a DIY hot tub don’t like it.

• Acid washing plaster pools. This is a professional job on plaster (not tiled) pools, done with the pool drained. It removes a thin layer of surface, so it isn’t something to do regularly.
• Efflorescence and cement stains on block or brick. Diluted acid is the traditional brick cleaner, but wet the surface first, test a small area and rinse thoroughly.
• Tiles and grout: be very careful. Acid attacks cement-based grout and render and will etch natural stone like limestone and marble. On a tiled DIY tub, use a proper tile and grout cleaner or scale remover instead. See hot tub tiles and waterproofing, grout and tile adhesive.
• Filter cartridges. Acid soaks are sometimes used to remove scale from pool cartridges, but only after degreasing, because acid “sets” oils into the pleats. For a hot tub cartridge, a proper filter cleaner is simpler and safer. See how to clean a hot tub filter cartridge.
• Not on stainless steel. Don’t use hydrochloric acid on ladders, handrails, light rims or other stainless fittings. Chloride attacks stainless and causes pitting and rust.

Common mistakes

• Adding water to acid instead of acid to water.
• Chasing pH before fixing alkalinity, then going round in circles. Fix TA first.
• Dumping a pool-sized dose into a hot tub and crashing pH. Small doses, retest.
• Pouring acid into the skimmer or filter compartment. It passes neat through the pump and heater.
• Storing acid next to chlorine or the spa pack.
• Using the same scoop, bucket or jug for acid and chlorine.
• Getting in too soon. Retest first.

Need parts or supplies?

If low pH has already eaten a heater element or pump seal, you’ll find replacements at parts4tubs.co.uk in the UK and parts4tubs.com in the USA. For the wider routine, see my beginner’s guide to hot tub maintenance, and if your water’s cloudy, cloudy hot tub water. And never mix acid with shock. See hot tub shock for how to do that safely.

FAQs

Is muriatic acid the same as hydrochloric acid?

Yes. Muriatic acid is hydrochloric acid. The name is mainly used in the US pool trade, where pool-grade products are typically around 31% strength.

Can I use muriatic acid in a hot tub?

Yes, in small, diluted doses, with the jets running and the lid off, retesting as you go. Most hot tub owners find granular pH minus (sodium bisulfate) easier and safer to handle.

Can I buy hydrochloric acid in the UK?

Only at 10% or below without a licence. Since 1 October 2023, members of the public need a Home Office EPP licence to buy hydrochloric acid above 10% w/w. For a hot tub, use pH minus instead.

Do you add acid to water or water to acid?

Always add acid to water. Fill the bucket with water first, then pour the acid in slowly.

How long after adding acid can I use my hot tub?

Once it’s circulated for at least 20–30 minutes (or the time on the label) and a retest shows pH and TA in range. Follow the product label.

Does muriatic acid lower alkalinity or pH?

Both. To bring TA down while keeping pH in range, dose acid, then aerate with the jets and blower to lift pH back up without raising TA.

Got a pH or alkalinity reading that won’t behave? Drop it in the comments below with your numbers. I read them all.

Cheers,
Andi

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Hi, Andi here. I own Buildahottub.com and also write all of the articles and info pages on the site. Some years back now, I built my own hot tub but struggled to find the information I needed. So, once my tub was complete, I started this website to help others in their own pursuit of hot tub and plunge pools DIY building information.

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