Hot tub or small spa
You have a few hundred to about 1,500 litres and you want a small tub, not an 18 kg bucket. Zagosa 1.5 kg is about £10.95, with 4.8 from 13 reviews and three UK texts that say it works. The item type name still says 1 kg.
pH minus comparison 2026
The best pH minus granules, compared.
Top pick: Deep Blue pH minus, 1 kg Best value: Zagosa pH minus granules, 1.5 kg – About £10.95, 4.8 from 13 reviews, and three UK texts that say it lowered pH. The item type name still says 1 kg. Updated October 2026.
PH minus is the dry acid you add when a test shows pool or hot-tub water is too alkaline. Where a listing names the chemical, it is sodium bisulphate, which also pulls total alkalinity down and leaves sulphate in the water. A usable tub has a pack weight that agrees with itself, a printed dose, and a review line. Cheaper 2you tubs name sodium bisulphate and print a dose, and they have no review line. BISHTA's domestic hot-tub band is pH 7.0–7.6, with a reducer only when the reading is above 7.6. The [pH minus buying guide](/hub/pool-care/ph-minus/sub/ph-minus-buying-guide) sets out pack size, unnamed chemicals, and listings whose weights do not agree. On these amazon.co.uk listings the usual product is granules. Low pH needs pH plus. pH minus is not a sanitiser and does not replace chlorine or bromine. One 2you listing is 500 g of pH minus and 500 g of pH plus, so half the pack raises pH if your test is already high.
PH minus is the dry acid you add when a test shows pool or hot-tub water is too alkaline.
38 products checked · 58 hrs invested · +18,600 readers
11 of 11: Best pH minus granules compared
Swipe for more
1-2 of 11Best overall
Best value
3.2/10
2.8/10
6.2/10
8.1/10
4.3/10
4.0/10
5.4/10
5.0/10
4.7/10
2.4/10
1.6/10
∼ £16
∼ £10
∼ £11
∼ £18
∼ £27
∼ £68
∼ £11
∼ £6
∼ £8
∼ £32
∼ £19
5.0(1)
5.0(1)
4.8(13)
4.7(151)
4.4(169)
4.4(25)
4.4(18)
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Personal buying guide
Which pH minus fits this water?
Question 1 of 2
What are you treating?
The best pH minus granules in detail
Our view
Deep Blue's 1 kg granules are £15.94, with a printed target of 7.2–7.6 for a pool or hot tub. The line is 5.0 from 1 review, and no review text is in the file. The chemical is not named and no gram dose is printed. A 1 kg tub at this price sits above Zagosa's 1.5 kg at £10.95, which has 13 reviews, and well above the price per kilogram of Zagosa's 5 kg at £18.25. The listing is coherent on weight. The single star does not tell you whether the granules dissolved.
Pros
- 1 kg title, weight and unit count agree
- Target band 7.2–7.6 is printed
- Bullets name a pool or a hot tub
Cons
- 5.0 from 1 review, with no text saved
- £15.94 for 1 kg, dear beside reviewed larger tubs in this set
- Chemical not named and no gram dose printed
Best for: You want a 1 kg tub whose weight fields match and you do not need a review file.
Less suitable: You are comparing on UK reviews or on grams of product per pound.
Specifications
- Model
- Deep Blue pH minus, 1 kg
- Brand
- Deep Blue
- Price
- ∼£16
- Customer rating
- 5.0/5(1)
- Pack size
- 1 kg
- Chemical
- Not named
- Suitable for
- Pool or hot tub
- Target p H
- 7.2–7.6
- Price per kg
- £15.94/kg
- Weight on listing
- Agrees
Common questions
Does the listing say it can be used in a hot tub?
The bullets say it decreases the pH of a pool or hot tub. A separate question on the page is not answered beyond that.
Is a dose printed?
No gram rate is in the saved listing. The target band printed is 7.2–7.6.
What do reviewers say?
The listing line is 5.0 from 1 review. No review text is on the listing.
Our view
The 2you set at £9.99 is 500 g of pH minus and 500 g of pH plus, not a pure reducer. The target printed is 7.2–7.6, the bullets mention a child-safe opening, and sodium bisulphate is not named. There is no gram dose. The rating is 5.0 from 1 review, with no text saved. If your test is already low, the plus tub is the half you need. If it is high, half the pack is the wrong direction. Buying it as pH minus means paying for an increaser you may not open.
Pros
- Both directions in one £9.99 pack if your tests swing either way
- Target 7.2–7.6 printed
- Child-safe opening claimed on the bullets
Cons
- Only 500 g is reducer; the other 500 g is pH plus
- Chemical not named and no gram dose
- 5.0 from 1 review and no saved text
Best for: Your tests move both ways and you want a small plus tub as well as a small minus tub.
Less suitable: You already know the water is high and you only want reducer.
Specifications
- Model
- 2you pH minus and pH plus set, 1 kg
- Brand
- 2you
- Price
- ∼£10
- Customer rating
- 5.0/5(1)
- Pack size
- 500 g + 500 g plus
- Chemical
- Not named
- Suitable for
- Spa, pool, paddling pool
- Target p H
- 7.2–7.6
- Price per kg
- £9.99/kg
- Weight on listing
- Half is pH plus
Common questions
Is this only pH minus?
No. The bullets say one 500 g tub of pH plus and one 500 g tub of pH minus.
Does the listing name sodium bisulphate?
No. The form is granular powder and the target band printed is 7.2–7.6. No gram dose is in the saved fields.
Is there a child-resistant lid?
The bullets say the tubs have a child-safe opening. No review text is saved. The line is 5.0 from 1 review.
Our view
Zagosa's 1.5 kg granules sit at £10.95 and 4.8 from 13 reviews. Three saved UK texts say the product works, and one of them says it brought a pool's alkalinity and pH down. None of those three describes a leak, a smell, or a failed dose, though the 13-review line is larger than the texts on file, so other faults are unknown. The title, model and unit count say 1.5 kg, while the item type name says 1 kg. The chemical is not named and no gram dose is printed. For a hot tub or a small top-up, this is the best value in the set among packs a UK review says actually moved pH, if you treat the 1 kg item-type line as unfinished.
Pros
- £10.95 for a 1.5 kg unit count
- 4.8 from 13 amazon.co.uk reviews
- Three UK texts say it works, including one that lowered pool alkalinity and pH
- Sized for a spa or a short run of corrections rather than an 18 kg bucket
Cons
- Item type name says 1 kg while the unit count says 1.5 kg
- Chemical not named and no dose printed
- Only three review texts are saved against a 13-review line
Best for: You want a small reviewed tub under £12 for a hot tub or a light pool correction.
Less suitable: You need a named chemical, a printed dose, or a 5 kg pack with a much larger UK review file.
Specifications
- Model
- Zagosa pH minus granules, 1.5 kg
- Brand
- Zagosa
- Price
- ∼£11
- Customer rating
- 4.8/5(13)
- Pack size
- 1.5 kg
- Chemical
- Not named
- Suitable for
- Hot tub, spa, pool
- Price per kg
- £7.30/kg
- Weight on listing
- Type name says 1 kg
Common questions
Is the tub 1.5 kg or 1 kg?
The title, model and unit count say 1.5 kg. The item type name says Zagosa Spa & Pool 1 kg pH Minus.
Does it claim to lower alkalinity as well as pH?
Yes. The title calls it an alkalinity reducer, and the bullets say it reduces pH and alkalinity. No gram dose is printed.
What do the saved reviews say?
Three UK texts say it works, including one that lowered pool alkalinity and pH. None of those three describes a fault. The line is 4.8 from 13 reviews.
Our view
The Zagosa 5 kg tub is the listing in this set with a UK review file you can actually read: 4.7 from 151 amazon.co.uk reviews, and 15 of the saved texts are labelled United Kingdom. Buyers say the granules dissolve, that a pool came round within a couple of days, and that the quantity lasts when pH runs high. One of those texts says the container leaks, so the tub is not a sealed story. The unit count is 5,000 g, yet one bullet still calls it a 1 kg tub, and the saved fields do not name sodium bisulphate or print a gram dose. At £18.25 it is the lowest price here for a 5 kg pack that UK reviewers describe using. It is the top pick for a season of pool corrections, with that weight bullet still unresolved.
Pros
- 4.7 from 151 amazon.co.uk reviews, with 15 UK texts saved
- UK reviewers say it dissolves and that a pool came round within a couple of days
- £18.25 for a 5 kg unit count, the lowest 5 kg shelf price in this set
- Reviewers also describe it lowering alkalinity with pH
Cons
- One bullet still says this is a 1 kg tub
- Sodium bisulphate is not named and no gram dose is printed
- One UK reviewer wrote that the container leaks
Best for: You want a 5 kg season pack for a pool, and you can live with a listing that still contains a 1 kg bullet.
Less suitable: You need the chemical named on the page, a printed gram dose, or a tub small enough for a single spa top-up.
Specifications
- Model
- Zagosa pH minus granules, 5 kg
- Brand
- Zagosa
- Price
- ∼£18
- Customer rating
- 4.7/5(151)
- Pack size
- 5 kg
- Chemical
- Not named
- Suitable for
- Spa, hot tub, pool
- Price per kg
- £3.65/kg
- Weight on listing
- Bullet says 1 kg
Common questions
Is this tub 1 kg or 5 kg?
The unit count is 5,000 g and the item weight is 5 kg. One bullet still says this is a 1 kg tub. The listing does not resolve that.
Does the listing name the chemical?
No. The form field says granules. Sodium bisulphate is not printed in the saved fields.
What do the saved UK reviews say?
Buyers say it dissolves and that it lowered pH, including with alkalinity. One buyer wrote that the container leaks.
Our view
HTH's 5 kg microbeads are £26.95 and show 4.4 from 169 reviews, the largest count in the set. The 31 saved texts are labelled France, Germany and Spain, not the United Kingdom, so that count is not a UK sample. Those texts praise a small amount that dissolves, and they also repeat split buckets, loose powder, and lost product, including reports of about half the pack missing. A measuring cup is in the bullet. The chemical is not named, and the saved bullets do not say hot tub or pool. You pay more than Zagosa's 5 kg at £18.25 for a brand line whose damage stories are not from UK reviewers.
Pros
- 4.4 from 169 reviews on the listing line
- Saved texts say a small amount dissolves and lowers pH
- Bullet includes a measuring cup
- 5 kg unit count for a season pack
Cons
- Saved review texts are not labelled United Kingdom
- Repeated transit damage, with powder lost from split buckets
- Chemical and vessel are not named
- £26.95, above the other reviewed 5 kg tub in this set
Best for: You specifically want HTH microbeads and you will check the bucket before you store it.
Less suitable: You want UK review texts, a named chemical, or the lower 5 kg shelf price in this set.
Specifications
- Model
- HTH pH- microbeads, 5 kg
- Brand
- HTH
- Price
- ∼£27
- Customer rating
- 4.4/5(169)
- Pack size
- 5 kg
- Chemical
- Not named
- Price per kg
- £5.39/kg
- Weight on listing
- Item weight 5.25 kg
Common questions
Does the listing name the pool or hot tub?
No. Hot tubs and all pool types are questions on the page, and the saved bullets do not answer them.
Is a measuring cup included?
The bullet says the pack comes with 1 measuring cup. No gram dose is printed.
Are the 169 reviews from the UK?
The listing line is 4.4 from 169. The saved texts are from France, Germany and Spain. Several describe split buckets and lost powder.
Our view
Bayrol's listing is an 18 kg granule pack at £68.30, rated 4.4 from 25 reviews. All six saved texts are labelled Germany. Two of them say three 6 kg buckets arrived instead of one 18 kg bucket, and one describes a broken bucket. Short comments still call the goods fine. The bullets claim easy dissolving, high purity, and less extra corrosion than hydrochloric acid, which is the seller's sentence, not a test of this ASIN. The chemical is unnamed and no dose is printed. It is a bulk buy only if you can store that weight and you accept the pack-format dispute.
Pros
- 18 kg on the title and unit count, the largest pack in the set
- £68.30, a low price per kilogram if the full weight arrives
- Bullets claim easy dissolving and high purity
- 4.4 from 25 reviews, with short comments that the goods were fine
Cons
- Two Germany-labelled reviews say three 6 kg buckets arrived
- A broken bucket is in the saved texts
- Saved reviews are not UK-labelled
- Chemical not named and no dose printed
Best for: You run a pool that will use 18 kg and you can store several buckets if that is what arrives.
Less suitable: A hot tub, a paddling pool, or anyone who needs one confirmed 18 kg bucket and UK reviews.
Specifications
- Model
- Bayrol pH-minus granules, 18 kg
- Brand
- Bayrol
- Price
- ∼£68
- Customer rating
- 4.4/5(25)
- Pack size
- 18 kg
- Chemical
- Not named
- Price per kg
- £3.79/kg
- Weight on listing
- Reviews: 3 × 6 kg
Common questions
Does the listing say it arrives as one 18 kg bucket?
The title and unit count say 18 kg and one item. Two Germany-labelled reviews say three 6 kg buckets arrived instead.
Does it claim to prevent algae?
The page asks that. The saved bullets do not answer it. They say easy to dissolve, high chemical purity, and less extra corrosion than hydrochloric acid.
Where are the saved reviews from?
The line is 4.4 from 25. The six saved texts are labelled Germany. Themes are pack format and a broken bucket, not a failed dose.
Our view
Relax's 1.5 kg hot-tub pack is £11.00, with 4.4 from 18 reviews and two UK texts saved. One reviewer calls it good value and expects about 60 g per balance to last years on a Palm Springs tub of about 950 litres. The same text says mixing water that was too warm gave off a cough-inducing vapour. The catalogue form field says Gel, while that review describes dissolving a measured dose in warm water, and the chemical name is not in the file. There is no printed gram rate. It is a small-spa option beside Zagosa's 1.5 kg tub, with a form field you should not silently rewrite and a vapour note if you stir it hot.
Pros
- £11.00 for 1.5 kg, aimed at a hot tub in the title
- 4.4 from 18 reviews, with two UK texts calling it good value
- One UK reviewer dissolved a measured dose and reported a lasting bottle
Cons
- Catalogue form field says Gel
- Chemical not named and no dose printed on the listing
- One reviewer reported cough-inducing vapour when the mixing water was too warm
- Only two texts are saved against an 18-review line
Best for: You want a 1.5 kg hot-tub pack near £11 and you will dissolve it in warm, not hot, water.
Less suitable: You need granules stated as granules, a named chemical, or a larger UK review sample.
Specifications
- Model
- Relax spa pH minus, 1.5 kg
- Brand
- Relax
- Price
- ∼£11
- Customer rating
- 4.4/5(18)
- Pack size
- 1.5 kg
- Chemical
- Not named
- Suitable for
- Hot tub
- Price per kg
- £7.33/kg
- Weight on listing
- Agrees
Common questions
Why does the form say Gel?
The catalogue field says Gel. A UK review describes mixing a measured dose into warm water. The chemical name is not on the listing.
Is a dose on the listing?
No. One reviewer, not the listing, used about 60 g on a Palm Springs hot tub of about 950 litres.
What did UK reviewers report?
Two saved texts call it good value. One says water that was too warm while stirring gave off a cough-inducing vapour. The listing line is 4.4 from 18 reviews.
Our view
The 2you 500 g reducer is £5.97 and is one of only two listings here that print sodium bisulphate in the bullets. The seller says 10 g per 1,000 litres lowers pH by 0.2, names a 7.2–7.6 target, and describes a press-down lid. There is no amazon.co.uk review line in the file, so nobody in this batch has confirmed the dose or the lid. The pack is a spa-sized top-up, not a season's pool supply. You get a named dry acid and a printed rate at the lowest shelf price in the set, and you also get no customer evidence at all.
Pros
- £5.97, the lowest shelf price in this set
- Bullets name sodium bisulphate
- Seller dose printed: 10 g per 1,000 litres for a 0.2 drop
- Press-down lid described on the listing
Cons
- No amazon.co.uk review line in the file
- 500 g will not cover a season of pool corrections
- The 0.2 claim is the seller's bullet, not a result measured here
Best for: You want a small named dry-acid top-up and you accept a listing with no reviews.
Less suitable: You want UK reviewers to have described the dose, or you need several kilograms for a garden pool.
Specifications
- Model
- 2you pH reducer granules, 500 g
- Brand
- 2you
- Price
- ∼£6
- Pack size
- 500 g
- Chemical
- Sodium bisulphate
- Suitable for
- Spa, pool, paddling pool
- Target p H
- 7.2–7.6
- Price per kg
- £11.94/kg
- Weight on listing
- Agrees
Common questions
What chemical does the listing name?
The bullets call it dry acid sodium bisulphate. That is the seller's wording, not a lab result.
What dose is printed?
The bullet says 10 g per 1,000 litres lowers pH by 0.2, and that a dose lowers alkalinity by 10–20 ppm. The target printed is 7.2–7.6.
Is the lid described as child-resistant?
Yes. The bullet describes a press-down lid that does not open by twisting. There is no customer-review line on the listing.
Our view
This 2you listing is £8.49 for 1 kg sold as two 500 g tubs, and the bullets again name sodium bisulphate. The same seller rate appears: 10 g per 1,000 litres for a 0.2 drop, plus a 10–20 ppm alkalinity line, with a 7.2–7.6 target. The title says 1 kg, the unit count is 1,000 g, and the number of items is 2, so the split is at least consistent. There is still no review line. You pay more than the single 500 g tub for a second container of the same powder, which helps if you want a spare, and does nothing to prove the dose.
Pros
- Sodium bisulphate named on both the chemical and the dose bullet
- Two 500 g tubs, so one can stay sealed
- Title, unit count and item count agree on 1 kg as two tubs
- £8.49, still in the under-£10 band
Cons
- No review line
- More expensive per tub than the single 500 g listing
- The 0.2 and 10–20 ppm lines are seller bullets
Best for: You want two small named tubs rather than one 500 g pot, and reviews are not your filter.
Less suitable: You want customer texts, or a single 5 kg bucket for a pool season.
Specifications
- Model
- 2you pH reducer, 2 × 500 g
- Brand
- 2you
- Price
- ∼£8
- Pack size
- 2 × 500 g
- Chemical
- Sodium bisulphate
- Suitable for
- Pool, spa, paddling pool
- Target p H
- 7.2–7.6
- Price per kg
- £8.49/kg
- Weight on listing
- Two tubs, 1 kg
Common questions
Is this one tub or two?
The title says 1 kg. The bullets say two 500 g tubs. Unit count is 1,000 g and the number of items is 2.
What dose is printed?
10 g per 1,000 litres lowers pH by 0.2, and a dose lowers alkalinity by 10–20 ppm. Both lines are seller bullets. The target printed is 7.2–7.6.
What if pH is already low?
The page asks that question and does not answer it. This listing is a reducer, so it is the wrong product when pH needs to rise.
Our view
Waterman's 5 kg granules are £32.12, the dearest 5 kg tub in this set. The bullets say they dissolve without residue and suit all swimming pools. Hot tubs are not named, the chemical is not named, and no dose or target pH is printed. There is no review line. Zagosa's 5 kg listing is £18.25 with 151 reviews. You would be paying almost twice that for a coherent 5 kg weight and a pool-only sentence, with no customer file to check.
Pros
- 5 kg title and item weight agree
- Bullets say the granules suit swimming pools and dissolve without residue
Cons
- £32.12, the highest 5 kg price in this set
- No review line, no named chemical, and no dose
- Hot tubs are not mentioned
Best for: You want a 5 kg pool-only tub and the brand name matters more than reviews or price.
Less suitable: A hot tub, or anyone comparing this with the cheaper reviewed 5 kg listing.
Specifications
- Model
- Waterman pH minus granules, 5 kg
- Brand
- Waterman
- Price
- ∼£32
- Pack size
- 5 kg
- Chemical
- Not named
- Suitable for
- Swimming pools
- Price per kg
- £6.42/kg
- Weight on listing
- Agrees
Common questions
Does the listing name a hot tub?
No. The bullets say suitable for all swimming pools. Hot tubs are not mentioned.
Is a dose printed?
No. The bullets say easy dosage, quick dissolve, and no residue. They do not give grams or a target pH.
Is there a star rating?
No customer-review line is on the listing. The chemical is not named.
Our view
Shatchi's listing is £18.60 and does not settle how much product is in the box. The title says a pack of 2, the unit count is 2,000 g, the item weight is 3 kg, and the include line says 1.5 kg. The bullets say dissolve the recommended amount in warm water, wait at least 4 hours, and aim at 7.2–7.6. No gram rate is printed, the chemical is not named, and there is no review line. Until those weights agree, a price per kilogram is not a number you can use. The warm-water and four-hour lines are the useful part of the page, and they do not tell you what you are paying for.
Pros
- Target 7.2–7.6 is printed
- Bullets say to dissolve in warm water and check after at least 4 hours
Cons
- Title, unit count, item weight and include line do not agree
- No review line and no named chemical
- No gram dose, only the recommended amount
Best for: Only after the seller confirms one fill weight, if you still want this listing.
Less suitable: Anyone who needs to know whether they are buying 1.5 kg, 2 kg or 3 kg.
Specifications
- Model
- Shatchi pH minus granules
- Brand
- Shatchi
- Price
- ∼£19
- Pack size
- Unresolved
- Chemical
- Not named
- Suitable for
- Hot tub, paddling pool
- Target p H
- 7.2–7.6
- Weight on listing
- Fills do not agree
Common questions
How much product is in the pack?
The title says a pack of 2. The unit count is 2,000 g, the item weight is 3 kg, and the include line says 1.5 kg. The listing does not reconcile those figures.
How does the listing say to add it?
Dissolve the recommended amount in warm water, add the solution, and check after a minimum of 4 hours. No gram rate is printed.
Is there a review line?
No customer-review line is on the listing. The chemical is not named.
How we compared
Eleven pH minus listings on amazon.co.uk were scored from listing facts dated 19 October 2026, not from a lab test. Amazon evidence, listing fit, review outcome and value are weighted 30, 35, 20 and 15. PWTAG and BISHTA set the safety frame. They are not a score for any ASIN. The highest score is the top pick. The best shelf price among the smaller tubs that UK reviews say actually lowered pH is the best value. One of each.
1
Amazon evidence
30%Star rating and review count on amazon.co.uk. Points come off when the line is missing, when the count is one review, or when the saved texts are not from the United Kingdom.
2
Listing fit
35%A coherent pack weight, a named chemical, a printed dose, and a vessel the listing actually names. Contradictory fills, and a plus-and-minus twin sold as a reducer, lose points.
3
Review outcome
20%UK texts that describe pH actually moving score higher than a large file of transit damage, or a file with no texts at all.
4
Value
15%Shelf price in GBP against a pack size a UK owner can use. A low price per kilogram does not score when the fill weight is unresolved or the reviews say a different pack arrived.
Independent review
Last updated: October 2026
8 sources checked
pH minus buying guide
Start with the test, then the pack weight, then whether a UK review says the pH actually moved. Price per kilogram comes after those three. The notes below follow that order for these eleven amazon.co.uk listings. None of the scores is a dissolve test in a lab.
What is pH minus?
pH minus is the dry acid you add when a test shows the water is too alkaline. On these amazon.co.uk listings the usual product is granules. Where a listing names the chemical, it is sodium bisulphate. That also pulls total alkalinity down and leaves sulphate in the water. Low pH needs pH plus. pH minus is not a sanitiser and does not replace chlorine or bromine. One 2you listing is 500 g of pH minus and 500 g of pH plus, so half the pack raises pH if your test is already high.
Best for
- A hot tub of a few hundred to about 1,500 litres, where a 500 g to 1.5 kg tub is enough
- An above-ground pool that needs a 5 kg season pack and a dose from that label
- A buyer comparing dry acid with hydrochloric acid, including a tiled pool where grout is at risk
Not ideal for
- Anyone whose test is already low, because pH minus will push it lower
- A paddling pool, if the object in the basket is Bayrol's 18 kg listing
- A buyer treating HTH's 169-review line as United Kingdom opinion
Mistakes to avoid
- 1
Dosing with no current test
pH minus is for water that is already high. A low reading needs pH plus. BISHTA's hot-tub reducer line starts above 7.6. Pool labels in this set more often print 7.2–7.6. Retesting too soon does not show the dose.
- 2
Pouring dry granules onto a liner
Add the chemical to water in a clean plastic bucket. Shatchi's bullets say dissolve the recommended amount in warm water and check after at least 4 hours. A Relax review reported cough-inducing vapour when the mixing water was too warm.
- 3
Mixing the powder with chlorine
HSE notes that mixing acid with hypochlorite can release chlorine gas. Store the tub dry, away from chlorine products and children. pH minus does not replace a sanitiser. Call a professional for a commercial tiled pool, an eye injury, or a suspected gas incident.
What to weigh up
Pack weight, chemical and dose
HighTrust a fill only when the title, unit count and item weight agree. Zagosa's 5 kg unit count is 5,000 g, and one bullet still says 1 kg tub. The 1.5 kg Zagosa item type name says 1 kg. Shatchi's title says a pack of two, the unit count is 2,000 g, the item weight is 3 kg, and the include line says 1.5 kg. Bayrol's title says 18 kg, and two Germany-labelled reviews say three 6 kg buckets arrived. HTH's unit count is 5 kg and the item weight is 5.25 kg.
Pack size, reviews and price
HighA 500 g to 1.5 kg tub suits a spa of a few hundred to about 1,500 litres. A 5 kg tub suits a season of pool corrections. Bayrol at about £68.30 is an 18 kg object and the wrong buy for a paddling pool. In this batch, under £10 covers 2you 500 g at about £5.97, 2you 1 kg as two tubs at about £8.49, and the plus-and-minus set at about £9.99. Zagosa 1.5 kg is about £10.95 and Relax 1.5 kg is about £11.00. Zagosa 5 kg is about £18.25. HTH 5 kg is about £26.95 and Waterman 5 kg is about £32.12.
Which option suits you?
Three typical situations, each with a recommendation from this comparison.
Above-ground pool for a season
You want a 5 kg pack for repeated corrections, and you can live with a listing that still contains a 1 kg bullet. Zagosa 5 kg is about £18.25, with 4.7 from 151 reviews. UK texts say it dissolves. The chemical is not named.
Named dry acid, no review file
You want the bullets to say sodium bisulphate and to print a dose, and you accept a listing with no reviews. 2you 500 g is about £5.97 and prints 10 g per 1,000 litres for a 0.2 drop. That rate is the seller's bullet. The pack will not cover a pool season.
Common questions about pH minus granules
Which pH minus is the top pick in this set?
The top pick in this set is Zagosa pH minus granules, 5 kg (B0B5YJ53C8), at about £18.25 on amazon.co.uk. The line is 4.7 from 151 reviews, and 15 saved texts are labelled United Kingdom. Buyers say the granules dissolve and that a pool came round within a couple of days. One text says the container leaks. A bullet still calls it a 1 kg tub. The listing does not name sodium bisulphate or print a gram dose. The score uses those listing facts. It is not a lab test.
Which pH minus is the best value?
The best value is Zagosa pH minus granules, 1.5 kg, at about £10.95. The line is 4.8 from 13 reviews. Three UK texts say it works, including one that lowered a pool's alkalinity and pH. The title, model and unit count say 1.5 kg, while the item type name says 1 kg. The chemical is not named and no gram dose is printed. It suits a hot tub or a light correction. It is the best shelf price among the smaller tubs that UK reviews say actually moved pH.
What should you check before you buy pH minus?
Confirm the test is high before you buy a reducer. Low pH needs pH plus. Then check that the title, unit count and item weight name the same fill. Zagosa's 5 kg listing still has a 1 kg bullet, and Shatchi's fill figures do not agree. Prefer a listing that names sodium bisulphate if you need to know it is dry acid. Read the dose on that tub. Seller rates in this set do not match, so 10 g per 1,000 litres is not a universal UK dose.
What pH should a home pool or hot tub sit at?
BISHTA's domestic hot-tub band is pH 7.0–7.6, and a reducer is for a reading above 7.6. Pool labels in this set more often print 7.2–7.6. PWTAG's chlorine guidance for managed pools sits nearer 7.0–7.4. Those are not the same rule. Test before you dose, keep a residual sanitiser, and do not treat pH minus as chlorine. Retest after the water has circulated. A reading taken too soon does not show whether the dose has worked.
Is pH minus a sanitiser?
No. pH minus lowers pH and, where the chemical is sodium bisulphate, it also lowers total alkalinity. It does not kill algae or bacteria, and it does not replace chlorine or bromine. Liquid hydrochloric acid and carbon dioxide are other pH correctants in PWTAG's notes. They are not the tubs in this amazon.co.uk set. If the job is free chlorine, that is a separate product. Compare chlorine granules for a sanitiser, not for pH.
How does sodium bisulphate differ from liquid acid?
Where a listing names it, sodium bisulphate lowers pH and alkalinity faster than hydrochloric acid and leaves sulphate in the water. PWTAG cites a 360 mg/l sulphate ceiling where cement grout can be attacked, and says to use sulphate-resistant cement when that risk applies. Hydrochloric acid is not in this set of tubs. Carbon dioxide is PWTAG's preference for many pools at or below about 150 mg/l alkalinity and 300 mg/l calcium hardness, and a poor fit for spas. None of these ASINs is PWTAG-approved.
When should you call a professional?
Call a professional for a commercial tiled pool, a suspected chlorine-gas incident, an eye injury, or a large pool that still will not move after doses that follow the label. HSE treats acids as COSHH substances in a commercial pool. Mixing acid with hypochlorite can release chlorine gas. At home, add the chemical to water in a clean plastic bucket, wear eye protection, and store the tub dry, away from chlorine products and children. A cited pool-acid safety data sheet lists Eye Dam. 1, H318, serious eye damage.
Are the largest review counts United Kingdom reviews?
HTH's listing line is 4.4 from 169 reviews, the largest count here. The saved texts are labelled France, Germany and Spain, not the United Kingdom, so that count is not a UK sample. Those texts include split buckets and lost powder. Bayrol is 4.4 from 25, and the six saved texts are labelled Germany. Two of them say three 6 kg buckets arrived instead of one 18 kg bucket. A star line is the amazon.co.uk figure. It is not, by itself, United Kingdom opinion.
Our verdict
In this set of eleven amazon.co.uk listings, the top pick is Zagosa pH minus granules, 5 kg (B0B5YJ53C8). It is about £18.25, with 4.7 from 151 reviews and 15 UK texts that say the granules dissolve and last. One text says the container leaks, a bullet still says 1 kg, and the chemical is not named. The best value is Zagosa pH minus granules, 1.5 kg, at about £10.95, with 4.8 from 13 reviews and three UK texts that say it works. The other nine have no award. None of these scores is a lab test.
Best value
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Zagosa pH minus granules, 1.5 kg
About £10.95, 4.8 from 13 reviews, and three UK texts that say it lowered pH. The item type name still says 1 kg.
Editorial
Editor
DIY & toolsPool careamazon.co.uk listings
Peter F. is the editor for DIY & tools at productscompared.co.uk. Comparisons in this category are checked against amazon.co.uk listings, review evidence and public UK sources.
Updated: October 2026
8 sources checked
Guides to pH minus granules
Guides on what to look for, how to use a product, and common mistakes.
pH minus buying guidePack size, unnamed chemicals, and listings whose weights do not agreeBest pH minusWhich listing fits a spa, a 5 kg pool tub, or a bulk buyHow to dose pH minusPre-dissolve, circulate, retest, and do not mix with chlorinepH minus for a hot tubBISHTA's band, a small volume, and pH that will not stay downpH minus or liquid acidDry acid against hydrochloric acid, sulphate against chloride
Related comparisons
Sources
Public UK sources used for this page in October 2026. Shelf prices and star lines are the amazon.co.uk figures for these eleven listings.
- PWTAG: Choosing an acid pH correctant (retrieved 21 October 2026)
- PWTAG: Carbon dioxide for pH control (retrieved 21 October 2026)
- PWTAG: Sulphate attack TN3 (retrieved 21 October 2026)
- HSE: Swimming pools (retrieved 21 October 2026)
- HSE: Managing health and safety in swimming pools (HSG179) (retrieved 21 October 2026)
- BISHTA: Water treatment (retrieved 21 October 2026)
- BISHTA: Daily, weekly, monthly and annual hot tub checks (retrieved 21 October 2026)
- All Swim: Pool acid safety data sheet (retrieved 21 October 2026)
This page contains affiliate links to amazon.co.uk. If you buy through them, we may earn a commission at no extra cost to you. Prices, stars and review counts are the UK listing figures used for this comparison. We did not lab-test these pH minus tubs.