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UV clarifier for green pond water

Updated: 22 October 2026

The pond looks like pea soup and you are not sure a lamp is the right buy. A UV clarifier for green pond water is aimed at one growth: the single-celled algae suspended in water that actually passes the tube. It is the wrong tool for blanketweed, for brown silt, and for a filter that is not catching waste. Evolution Aqua’s window is about 3 to 14 days when the size is right, and up to six weeks when it is not. Reviews here mention three days to about ten, the short end of that window. The four listings are on the [UVC pond clarifier comparison](/hub/garden-ponds/uvc-pond-clarifier). A qualifying purchase can earn us a commission at no extra cost to you.

The pond looks like pea soup and you are not sure a lamp is the right buy. A UV clarifier for green pond water is aimed at one growth: the single-celled algae suspended in water that actually passes the tube.

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
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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.Green water is not blanketweed
  2. 2.What the lamp does to cells that pass through
  3. 3.When it is still green
  4. 4.What not to buy for this symptom

Green water is not blanketweed

Green water is free-floating cells. Blanketweed is the stringy growth on liner, plants and waterfalls. That Pond Guy and All Pond Solutions both separate the two, because a UVC only affects cells that enter the housing. A bigger lamp does not peel strands off the wall. Buying an evo110 for hairy margins spends £284.95 on the wrong symptom. Fill a clear jar and leave it for about 24 hours. Green that stays in the water is the UVC’s job. Brown that settles is silt. Strings you can lift on a stick are blanketweed, even if the water under them is green. You can have both. This page does not dose a bottled blanketweed treatment.

What the lamp does to cells that pass through

Water is pumped past a sleeved UV-C lamp. Cells that receive a clarifier dose clump, and the filter catches the debris. If the foams are blocked, the clumps stay in the pond. The maker’s no-residue line is for water that has passed a lamp kept inside the housing. It is not a licence to run the lamp in the open. The Health and Safety Executive’s warning on UV injury to eyes and skin is why the housing stays closed. If the unit is the right size, Evolution Aqua says you should see the pond clear in about 3 to 14 days, and that it can take up to six weeks. The conditions are a lamp on 24 hours a day, flow that is not too fast, the right volume, a new bulb from March, and a mechanical filter. The manual’s season is about mid-March to early October, unless the installation is frost-protected. Saved reviews, a few days on the evo25 and up to about ten on the evo55, are the short half of that window.
Evolution Aqua Evolution Aqua evoUV evo25

Testsieger

Evolution Aqua Evolution Aqua evoUV evo25
8.8/10about £129.99
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When it is still green

A glow is not a clean pond. evo55 reviewers look after dark because there is no useful daytime indicator. The glow means the lamp is lit, not that the quartz is clean or the bulb is young. The maker quotes about 9,000 hours and a spring replacement. Water Garden Ltd talks about 12 to 18 months. A bulb that still glows in its second summer can be under-dosing the pond. If the pump is faster than the model’s maximum, each litre spends too little time at the tube. An evo25 tops out around 10,000 litres per hour, an evo110 around 25,000. Full sun and heavy stock move a pond off the evo25 even when the litre maximum looks comfortable. One evo30 reviewer on a 4,500 litre pond was not fully clear after stepping up from 18 watts. That is a not-yet note, not proof you need the largest housing. If the sleeve went in wet, use the install guide before a second unit. If the litres were a guess, redo them on the buying guide.

What not to buy for this symptom

Liquid clarifiers and loose bulbs were thrown out of this comparison because the word clarifier in a title is not a UVC. A bottle is not the flow-through lamp these four listings are. A spare G23 compact bulb is the wrong part: the manual specifies a T8 with a G13 cap. A larger lamp is also the wrong next buy when the growth is blanketweed, when the filter is not removing waste, or when the sleeve is simply dirty. The evo110’s 110 watts will not strip strings off the liner. A slower, heavier dose than a green-water clarifier is a different job, on the koi pond page. For ordinary green water, give a correctly sized, clean unit the 3 to 14 days, and allow the six-week tail before you call it a failure.

Common questions

Will UV clear green water?
It can clear free-floating green algae that passes the housing, if the size is right and the filter catches the clumps. It does not clear blanketweed, and it does not replace a blocked filter. Green that stays in a jar after 24 hours is the symptom. Brown silt in the jar is a different job.
How long should I wait?
Evolution Aqua’s window is about 3 to 14 days when size, flow and bulb are right, and up to six weeks when they are not. Reviews on the evo25 and evo55 mention about three days to about ten, the short end only. Leave the lamp on 24 hours a day through that period.
Will it remove blanketweed?
No. Blanketweed is attached, so the strands often never enter the housing. A higher wattage does not wipe them off. UV is for the green tint in the water. If a stick lifts strings, every larger model in this set is still the wrong product.
The lamp glows and the pond is still green. What now?
Check the flow is under the model’s maximum, the quartz is clean, and the bulb is inside about 9,000 hours or a spring change. A dark housing in daylight can still be working. evo55 reviewers check the glow after dark. A new bulb will not fix a cell the grid has already left.
Is brown cloudy water the same job?
Usually not. Brown cloud that settles in a jar overnight is silt, which points at the filter or a clean-out. Green that stays mixed is the algae a clarifier is meant to clump. If you have both, the UV only addresses the green fraction.
Does UV harm fish and plants?
The maker’s no-harm line is for water that has passed a lamp kept inside the housing. UV-C injures the cornea and the skin, which is the Health and Safety Executive’s point. Fish in the pond are not sitting in the bare lamp. You still disconnect before you open the end cap.
Should the unit run all day?
Yes, 24 hours a day from about mid-March to early October, unless the installation is protected from frost. The note for a pond that will not clear repeats that run, with flow that is not too fast and a filter that can catch the debris. Switching it off each evening stretches the window.

Related pages

Blanketweed treatment comparison

UVC pond clarifier

Best UVC pond clarifier for the pond in front of you

What size UV clarifier for my pond, by sun and flow

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)