Koi pond UV clarifier size
Updated: 22 October 2026
You keep koi, or the pond is in full sun and heavily stocked, and a wildlife wattage looks thrifty until the water greens in May. Koi pond UV clarifier size, for the Evolution Aqua units on our [UVC pond clarifier comparison](/hub/garden-ponds/uvc-pond-clarifier), follows that maker’s full-sun and high-stock column. It does not follow a goldfish ratio, and it does not follow a dealer who says double it after the grid has already moved you. A part-shade goldfish pond stays on the [buying guide](/hub/garden-ponds/uvc-pond-clarifier/sub/uvc-pond-clarifier-buying-guide). The figures are the maker’s chart, checked against the listings from 19 October 2026. A qualifying purchase can earn us a commission at no extra cost to you.
You keep koi, or the pond is in full sun and heavily stocked, and a wildlife wattage looks thrifty until the water greens in May.
You keep koi, or the pond is in full sun and heavily stocked, and a wildlife wattage looks thrifty until the water greens in May. Koi pond UV clarifier size, for the Evolution Aqua units on our UVC pond clarifier comparison, follows that maker’s full-sun and high-stock column. It does not follow a goldfish ratio, and it does not follow a dealer who says double it after the grid has already moved you. A part-shade goldfish pond stays on the buying guide. The figures are the maker’s chart, checked against the listings from 19 October 2026. A qualifying purchase can earn us a commission at no extra cost to you.
Our top picks
| Badge | Product | When the grid points here | Maker maximum | Maximum flow | Note | Price | Offer |
|---|---|---|---|---|---|---|---|
| Best overall |  Evolution Aqua Evolution Aqua evoUV evo25 | Leave it in full sun with high stock | 20,000 L | 10,000 L/h | 8.8 | about £129.99 | View offer |
| Best value |  Evolution Aqua Evolution Aqua evoUV evo55 | Full sun and high stock under 20,000 L | 55,000 L | 20,000 L/h | 8.8 | about £199.99 | View offer |
 Evolution Aqua Evolution Aqua evoUV evo110 | Top cell, fittings bought separately | 75,000 L | 25,000 L/h | 8.6 | about £284.95 | View offer |
On this page
Why koi ponds step up a size
Koi produce more waste than a lightly stocked wildlife pond of the same litres, and full sun feeds the algae a clarifier is meant to knock back. Evolution Aqua’s coarse line is still about 2 watts per 1,000 litres. The grid on top of that line changes the model. Under 20,000 litres, full sun and high stock point at the evo55 rather than the evo25. From about 30,000 litres upward, the calmer cell is already the evo55, and the demanding cells are the evo75 or the evo110. The evo75 is not in this comparison. Water Garden’s koi line is about 4 watts per 1,000 litres, and FishLife’s ratio is about 5. They explain why a wildlife sum feels small. They are not a second law on top of the evoUV grid. A 12,000 litre pond in full sun with a serious stock sounds like an evo25 on the 2 watt line, about 24 watts. The grid’s cell for that pond is the evo55. Buying the £129.99 evo25 because it is the top pick would follow review count, not the column you are in. The evo30 is not the rescue: one not-yet note on a 4,500 litre pond is about time, not a koi size.
Watts per gallon is a different sum
Oaks Koi and Cotswold Koi talk in watts per 1,000 gallons. A UK gallon is 4.55 litres. A US gallon is about 3.79 litres, and using it makes the pond look smaller, so this page uses the UK gallon only. Ten watts per 1,000 UK gallons is about 2.2 watts per 1,000 litres, close to Evolution Aqua’s coarse line before anyone doubles it. Cotswold’s example is 10 watts per 1,000 gallons in shade and double in full sun, so a 3,000 gallon pond wants 30 watts in shade and 60 in sun. That is about 13,650 UK litres. Oaks doubles a manufacturer rating in full sun, so a 55 watt unit sold for 5,500 gallons becomes a 110 watt suggestion. Evolution Aqua’s grid already has a full-sun column, and it does not always double the watts. Double a cell that has already stepped from the evo25 to the evo55 and you are following two rules. Convert the gallons, then read the maker’s cell.
Clarifier dose is not a steriliser dose
A clarifier dose is what these evoUV units are sized for: enough UV to damage free-floating algae so the filter can catch it. A steriliser dose, in the way Oaks describes it, is slower and heavier, aimed at organisms that pass the lamp, not at a greener tint alone. This comparison has no separate steriliser row and does not promise pathogen control. Turnover is where the two clash. Evolution Aqua prefers the whole pond through the unit about every three hours, inside the model’s maximum. Cotswold’s hosted Yamitsu booklet talks about every 1½ to 2 hours, and Water Garden’s koi turnover can be as fast as every half hour to one hour. On the evo55 the ceiling is 20,000 litres per hour. On the evo110 it is 25,000. If those faster figures pass the maximum, the unit’s maximum wins. Oaks’ line that UV does not harm bacteria is about organisms that are not passing the lamp. Do not treat it as Evolution Aqua’s claim.
Which captured unit that leaves you
Inside this set the koi choice is usually the evo55 or the evo110, with a gap where the evo75 should be. The evo55, at £199.99, has a maximum of 55,000 litres, an optimum around 15,000 litres per hour and a ceiling of 20,000. It is 4.4 from 41 reviews, with no wattage on the listing and one report of rain in the ballast. That is the best value mark. The evo110, at £284.95, prints 110 watts from two 55 watt lamps and shares the evo75’s 75,000 litre maximum, while asking for about 20,000 litres per hour rather than the evo75’s 17,500. UK fittings are not in the box. Twelve reviews and a comment that says Ok are a thin file. If the honest cell is the evo75, the evo110 is a neighbour with a faster optimum, not a silent substitute. The evo25’s top pick follows the listing sort. Leave it for a full-sun koi pond. Plumbing, including the electrician’s stop line, is on the install page. Which row to skip for other reasons is on the best UVC pond clarifier page.
Common questions
How many watts per litre for koi on these units?
Evolution Aqua’s coarse check is about 2 watts per 1,000 litres, then the grid, which for full sun and high stock under 20,000 litres points at the evo55. Water Garden’s koi figure is about 4 watts per 1,000 litres, and FishLife’s is near 5. Use the evoUV grid for an evoUV. Do not average the three into a third wattage.
Should I double the rating if the pond is in full sun?
Oaks and Cotswold double a gallon-based rating for full sun. Evolution Aqua’s grid already has that column, and it does not always double the watts. Doubling a cell the grid has already stepped up applies two rules. A 3,000 UK gallon pond is about 13,650 litres. A US gallon would make it look smaller.
Is a clarifier the same as a steriliser?
Not in the way koi dealers use the words. A clarifier dose clumps green water for the filter. A steriliser dose is described as slower and heavier, for organisms that pass the lamp. These four listings are clarifiers. This page does not promise parasite control, and it does not tell you to guess a slower flow.
evo55 or evo110 for a large sunny pond?
Use the evo55 for the 55,000 litre class, with a pump between about 15,000 and 20,000 litres per hour. Use the evo110 at the 75,000 litre cell if you can run nearer 20,000 to 25,000 litres per hour. Fittings are extra, and the review file is 12 ratings. The evo75 is not captured here.
Why do dealers talk in gallons?
UK koi retail has long used gallons. A UK gallon is 4.55 litres, so 10 watts per 1,000 UK gallons is about 2.2 watts per 1,000 litres. The same phrase with a US gallon changes the sum. Convert, then return to litres, because the manual is in litres per hour.
Will UV harm the filter bacteria?
Oaks says a UV does not harm beneficial bacteria that are not passing the lamp. That is not a promise about bacteria that do go through, and it is not Evolution Aqua’s line. The clarifier still needs the filter to catch clumped algae. Do not run the lamp outside the sleeve.
Is the evo30 enough because a shop liked it?
A shop can rank the evo30 first on sales and returns. amazon.co.uk showed 3.8 stars from eight reviews, and the full-sun, high-stock column does not land on that model. The £20 saving against the evo55 is a poor reason to ignore the grid.
Related pages
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.
- Evolution Aqua evoUV (retrieved 19 October 2026)
- Evolution Aqua evoUV installation manual (retrieved 19 October 2026)
- Evolution Aqua support: recommended flow for an evoUV 55 (retrieved 19 October 2026)
- FishLife: how to choose UV pond clarifiers (retrieved 19 October 2026)
- HSE: optical radiation (retrieved 19 October 2026)
- Water Garden Ltd: how often to change a UV bulb (retrieved 19 October 2026)
- Electrical Equipment (Safety) Regulations 2016 (retrieved 19 October 2026)
- Approved Document P (retrieved 19 October 2026)