Why the Reviews Split the Way They Do

Across its lifetime this algaecide averages 4.5 stars from more than 7,400 ratings. The hundred most recent reviews paint a rougher picture: they average just under 4 stars, with 67 five-star scores against 21 one-star scores and very little in between. People either love this bottle or want their money back.

Dig into the one-star pile and it thins out quickly, though. A few are about delivery, a soggy box left in the rain or import charges into Ireland, nothing to do with the liquid itself. Of the rest, nearly every actual product failure falls into one of two camps: buyers who poured it into an already-green pool and watched nothing happen, and buyers who received old stock with most of its shelf life gone. Both are worth understanding before you order, because both are largely avoidable.

Dosed Weekly From First Fill: The Fan Club

Start with the happy majority, because it is a big one. The five-star reviews are dominated by people who treat this as a weekly habit rather than an emergency response. "Clean pool all summer no green" is the entire review from one five-star buyer, Jai Dove, and dozens of others say the same thing at slightly greater length.

James Read describes the routine: "I give my circular 12ft diameter x 3ft depth pool a good splash of this every weekend", and reports the pool stays algae free. Beckie, who buys a bottle every year for the hot tub, writes: "We used this all summer long and have no issues with green water or algae build up". The long-term users are the most convincing of all. Kelly has run a pool for around nine years and calls Clearwater the only brand that "works every year with out fail" (sic). ZeeKay, four years in with the brand: "Not once have I had to discard my water partway through the summer due to cloudy or green water."

The pattern is boring in the best way: dose regularly alongside your chlorine, keep the filter running, and the water stays clear. For an above ground pool that takes a full afternoon and a lot of metered water to drain and refill, that is the whole point of the product.

Rescuing Water That Has Already Turned

Now the other camp. Cath Carey Williams gave one star after trying to bring a green pool back: "Used the recommended amount- did nothing." The review ends: "Whole bottle. Still. Nothing." Stephen Jack, also one star, drained and refilled a 24,000-litre above ground pool, dosed as advised with chlorine and pH in range, and still watched it get greener by the day, ending up two litres deep into the product. The advice there: "believe me spend those extra pennies and go for kleenpool", a rival algaecide that reviewer had used before. Several more one-star reviews tell the same story of upping the dose on a green pool and seeing no change.

But the rescues are not all failures, and the successful ones share a method. Fontana, four stars, sums it up as "Does the job...eventually.": early algae in the Intex pipework cleared only after a heavy dose, roughly 20 times the normal amount, with the pump running for a week. Ionel Cristian Preda, also four stars, reports the "whole bottle did not work, but after 2 days the pool is clean of algae". The most useful review in the whole set comes from David, five stars, who revived a pool that "had stood all winter and was a bit of a swamp": "Follow the instructions and it works perfectly. If your algae is heavy or well established simply use a second full dose." Water top-up, plenty of chlorine, two doses, filter running, sorted.

The takeaway: this is a slow chemical, not a magic one. If the water has already turned, plan on shock chlorination first, a second full dose of this, and several days of continuous filtering. Kirstie's five-star route matches that playbook: "No need to empty your pool, just shock dose and wait a few hours and the green will be gone!" Some buyers rescued their water this way. Plenty who expected an overnight miracle did not.

Check the Date Before You Pour

One complaint comes up too often to ignore: eight of the hundred most recent reviews raise the manufacture or expiry date. The bottle states a shelf life of two years from date of manufacture (DOM), and buyers keep receiving stock with much of that window already gone. Mark P. received a bottle with "a DOM of 09/2021" that was nearly a year past its expiry by the time it arrived in 2024. Roe was sent a 2021-manufactured bottle that could not be used or, apparently, returned. A two-star buyer put it plainly: "Item delivered was over a year old and has an expiry of only 24 months from date of manufacture." Halor's one-star summary: "This is great however, it lasts 2 years from manufacture and I only have a few months to use it as it is old stock!"

This matters beyond principle, because an expired algaecide is a plausible explanation for at least some of the "did nothing" one-star reviews above. Gracie's pool kept turning green until the household checked the mostly rubbed-off expiry date and concluded the batch was out of date. Alimac92 could not find a date of manufacture printed anywhere, reported it "Did not clear the algae at all", and left advice worth repeating: "best to buy from a store where you can see the DOM or expiry date".

Our advice is less drastic than abandoning the listing: order it, but check the printed date the moment the bottle arrives, and send it straight back if the manufacture date is old or missing. Amazon returns are painless at that stage. They are much harder once half the bottle is in 10,000 litres of water.

Hot Tubs and Spas: Small Water, Fast Results

If you own a hot tub or spa rather than a pool, the picture brightens considerably. Small water volumes mean the dose is tiny, results show quickly, and the glowing reviews cluster here. Mandy Tonge's five-star review is a neat example of why that matters in a hosepipe-ban summer: "My hot tub water was going a bit green but after adding this product within 12hrs it was nice and clear". No draining, no refilling, no wasted water.

One five-star buyer uses it alongside chlorine specifically because the bottom of the tub "could become slimy in texture", and reports this bottle solves that. The most detailed hot tub story comes from no8_chedworth, who fired up a tub after winter storage and found white flakes floating everywhere: biofilm dislodged from a filter that had been stored damp. Used together with Clearwater's clarifier, "no joke within 24 hours the water was crystal clear!", and the same fill was still going eight weeks later. David Pointon keeps it shorter, calling it "Essential treatment for keeping the water quality in hot tub".

Cold water is the one caveat in this section: Paul flynn, one star, "Used it on a cold plunge as recommended and didn’t work". A single report is not a verdict, but if your use case is a cold plunge rather than a heated tub, temper expectations.

The Slug Trick Under the Decking

One review stands out for readers of this site in particular. Pixie does not use it in a pool at all: "I pour this into the pools of water on my fibreglass roof under the decking and it kills the algae. This is how we stop the annual slugfest." Slugs graze on algae, and a damp, algae-coated surface under decking is prime slug habitat, so cutting off the food supply thins the population. The same review notes the bottle "lasts us a year or two" used this way.

We would not suggest sloshing pool chemicals around beds and borders, and this is one reviewer's off-label experiment rather than anything Clearwater endorses. But for standing water on flat roofs, under decking or in a disused water feature that keeps turning green, a capful of algaecide is a tidy fix, and that kind of dual use makes a 1-litre bottle easier to justify in a small garden.

Verdict: A Preventer First, a Rescue Second

A few practical notes before the score. This is concentrated stuff: FlakyKoala confirms it works but warns "trust me, I used too much, just to be sure it worked and the smell is overpowering... but , it worked" (sic). Ruth agrees on dosage: "You don’t need much... Clears the water beautifully." Measure against your water volume using the instructions printed on the bottle rather than free-pouring. Two reviewers also report leaking bottles; lauren knocked a star off after the bottle leaked "despite the lid being on fully!" and made a mess of the pool box, so store it upright in a tray.

So where does the score land? The lifetime 4.5 stars reflects years of satisfied weekly users, and for that job we think it is fair: used as a preventer from first fill, this is a cheap, effective bottle that lasts a season or more in a pool and far longer in a hot tub. As a rescue remedy for water that has already turned it is closer to a 3-star product, slow at best and a blank at worst, especially if you are unlucky with old stock. Our own rating is 4 out of 5. Buy it with the pool rather than after the bloom, check the date on arrival, dose it weekly, and it will be one of the least dramatic purchases of your summer. Check today's price on Amazon below.