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What size UV clarifier for my pond

Updated: 22 October 2026

You have a tape measure, a pump that quotes litres per hour, and water that has gone green. What size UV clarifier for my pond is not settled by one watts-per-litre slogan. On the [UVC pond clarifier comparison](/hub/garden-ponds/uvc-pond-clarifier) the only rows kept are four Evolution Aqua evoUV listings, so this page sizes you onto that maker’s chart and then checks the pump against the model’s flow window. Other shops’ ratios are the wrong chart once the manual has moved you for sun or stock. Peter F. used the manual and the amazon.co.uk listings from 19 October 2026. A qualifying purchase can earn us a commission at no extra cost to you.

You have a tape measure, a pump that quotes litres per hour, and water that has gone green. What size UV clarifier for my pond is not settled by one watts-per-litre slogan.

Our top picks

Evolution Aqua Evolution Aqua evoUV evo25

Best overall

Evolution Aqua Evolution Aqua evoUV evo25
8.8/10about £129.99
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Evolution Aqua Evolution Aqua evoUV evo55

Best value

Evolution Aqua Evolution Aqua evoUV evo55
8.8/10about £199.99
View offer
Evolution Aqua Evolution Aqua evoUV evo110

Rank 3

Evolution Aqua Evolution Aqua evoUV evo110
8.6/10about £284.95
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On this page
  1. 1.Measure the pond before you look at watts
  2. 2.Three UK rules, and which one belongs to these units
  3. 3.Flow, not just watts
  4. 4.What this comparison can actually sell you

Measure the pond before you look at watts

Every rule here is per 1,000 litres, so a guessed depth ruins the sum before watts matter. FishLife’s method is length times width times average depth, in metres, times 1,000. A rectangle of 3 metres by 2 metres by 0.6 metres is 3,600 litres. Use averages on a kidney, because shelves hold less than the centre. A water-meter reading from the fill beats the tape. Write the litres down before you open a product page. The maker’s maxima run from 20,000 litres on the evo25 to 75,000 litres on the evo110. Those ceilings are not a promise that a sunny, heavily stocked pond can use the whole number.

Three UK rules, and which one belongs to these units

Three public UK rules do not agree. Evolution Aqua’s coarse line is 2 watts per 1,000 litres. Water Garden Ltd publishes about 1 watt per 1,000 litres for a lightly stocked ornamental pond, about 2 for goldfish, and about 4 for koi. FishLife’s rule of thumb is 20 watts per 4,000 litres. Averaging them is not a specification. Use the chart for the brand you are fitting. For an evoUV that is the shade-and-stock grid, and the 2 watt line is only the check under it. Full sun plus high stock moves a pond under 20,000 litres off the evo25 and toward the evo55. From about 30,000 litres upward, calmer cells are already the evo55, and harder cells go to an evo75 we do not list or to the evo110. The evo15 is on the maker’s range and is not among these four rows. Heavy koi stocking stays on the koi pond UV clarifier size page.
Evolution Aqua Evolution Aqua evoUV evo25

Testsieger

Evolution Aqua Evolution Aqua evoUV evo25
8.8/10about £129.99
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Flow, not just watts

The lamp only treats water that passes it. Evolution Aqua wants the whole pond through the unit about every two to three hours, and prefers about three. Ten thousand litres needs about 3,300 litres per hour, and that rate still has to sit inside the model. The manual puts the evo25 at about 7,500 litres per hour optimum and 10,000 maximum, the evo30 at about 12,500 and 15,000, the evo55 at 15,000 and 20,000, and the evo110 at 20,000 and 25,000. A pump at 18,000 litres per hour through an evo25 is outside that unit. Water Garden turns fish ponds over faster, about every one to two hours. If that faster rate would pass the evoUV maximum, the unit’s maximum wins. The manual also says not to use a pump above 2.5 bar.

What this comparison can actually sell you

On 19 October 2026 the prices were £129.99 for the evo25, £179.99 for the evo30, £199.99 for the evo55 and £284.95 for the evo110. Only the evo110 listing prints a lamp wattage, at 110 watts. Watts for the other three come from the manufacturer table. The maker gives the evo25 as 540 mm, and the evo30 and evo55 as 990 mm. The amazon evo30 row says 57 cm. Do not average those lengths. A bigger housing wants a faster optimum flow, and the evo110 manual says UK hose fittings are not in the box. Many shaded ponds under about 7,500 litres land on the unlisted evo15. If the green is string on the walls, read the UV clarifier for green pond water page before you spend. Once the cell is settled, how to install a pond UV clarifier is the next job.

Common questions

How do I turn metres into litres?
Multiply length, width and average depth in metres, then multiply by 1,000. A pond 3 metres by 2 metres with an average depth of 0.6 metres is 3,600 litres. Use the average, not the deepest point, and reduce the rectangle if the shape is a kidney or the shelves are wide. A water-meter reading from the day you filled the liner is the better figure if you still have it.
Which watt rule should I trust for an evoUV?
Use Evolution Aqua’s grid: about 2 watts per 1,000 litres as the coarse UK check, then a step up for full sun or heavy stock. Water Garden’s 1, 2 and 4 watts per 1,000 litres, and FishLife’s 20 watts per 4,000 litres, are other retailers’ rules of thumb. They disagree, which is why this page does not average them. A different brand needs that brand’s chart.
What is the difference between optimum flow and maximum flow?
Optimum is the rate the maker wants for a proper dose. Maximum is the ceiling you should not pass. On the evo55 those figures are 15,000 and 20,000 litres per hour. A pump above the maximum shortens the time each litre spends at the lamp. The three-hour turn of 10,000 litres is about 3,300 litres per hour, and that figure does not cancel the model’s ceiling.
Is a bigger unit always safer?
No. A larger evoUV has a higher optimum flow and a higher price, and the evo110 does not include UK fittings according to the manual. The maker’s maximum litres are not a promise in full sun with heavy stock. Step up when the grid says so, not because the longest housing feels careful.
Why does the evo30 amazon size not match the brochure?
The saved amazon.co.uk row gives the evo30 as 57 × 8.8 × 16.7 cm. The manufacturer length for that model is 990 mm, the same body length quoted for the evo55. Those two numbers cannot both describe one housing. We leave the clash on the comparison and we do not overwrite the listing. If the parcel is a different length from the page you ordered, that is a description issue under the Consumer Rights Act 2015, not a figure to average at home.
What if my pump is above 2.5 bar?
The evoUV manual says not to use a pump above 2.5 bar. A waterfall pump can still be too much pressure for this sleeve and these hosetails. Kinking the hose to fake a lower pressure is a leak waiting for the first warm week. Split the flow or choose a feed the manual allows, then wet-test with the lamp out.

Related pages

Best UVC pond clarifier

UVC pond clarifier

UV clarifier for green pond water, not blanketweed

How to install a pond UV clarifier on the return pipe

Koi pond UV clarifier size for sun and heavy stock

Sources

Public UK and maker sources for this comparison. Listings and prices were taken from amazon.co.uk on 19 October 2026. IP65, wattages for the evo25, evo30 and evo55, and the flow figures are from the manufacturer manual, not from those three amazon pages.

  1. Evolution Aqua evoUV (retrieved 19 October 2026)
  2. Evolution Aqua evoUV installation manual (retrieved 19 October 2026)
  3. Evolution Aqua support: recommended flow for an evoUV 55 (retrieved 19 October 2026)
  4. FishLife: how to choose UV pond clarifiers (retrieved 19 October 2026)
  5. HSE: optical radiation (retrieved 19 October 2026)
  6. Water Garden Ltd: how often to change a UV bulb (retrieved 19 October 2026)
  7. Electrical Equipment (Safety) Regulations 2016 (retrieved 19 October 2026)
  8. Approved Document P (retrieved 19 October 2026)