Pond filter UV and green water
Updated: 22 October 2026
The pond has gone the colour of pea soup, the pads look dark, and you are about to buy a bigger box. A pond filter UV for green water only helps when the green is suspended algae in the flow, the lamp is actually producing UV, and the pump is running. Blanketweed is a different plant problem, and a dead bulb removes the clarifier even when the housing is fine. This page stays on that distinction and on which of these nine listings even claim a lamp. The GBP table is the [pond filter comparison](/hub/garden-ponds/pond-filter). Peter F. used the RHS, a retailer UVC note, and the 19 October 2026 amazon.co.uk snapshot. This is not a lab clearance test and not a blanketweed recipe. A qualifying purchase through our links can earn a commission at no extra cost to you.
The pond has gone the colour of pea soup, the pads look dark, and you are about to buy a bigger box.
The pond has gone the colour of pea soup, the pads look dark, and you are about to buy a bigger box. A pond filter UV for green water only helps when the green is suspended algae in the flow, the lamp is actually producing UV, and the pump is running. Blanketweed is a different plant problem, and a dead bulb removes the clarifier even when the housing is fine. This page stays on that distinction and on which of these nine listings even claim a lamp. The GBP table is the pond filter comparison. Peter F. used the RHS, a retailer UVC note, and the 19 October 2026 amazon.co.uk snapshot. This is not a lab clearance test and not a blanketweed recipe. A qualifying purchase through our links can earn a commission at no extra cost to you.
Our top picks
| Badge | Product | UV on the listing | Pump in the box | Layout | Note | Price | Offer |
|---|---|---|---|---|---|---|---|
| Best overall |  Fish Mate Fish Mate 2500 GUV | Yes, watts not stated | No | Gravity UV box | 9.0 | about £69.91 | View offer |
 Fish Mate Fish Mate 10000 PUV | Yes, on the bullets | No | Pressurised canister | 8.8 | about £146.75 | View offer | |
 Blagdon Blagdon Inpond 5-in-1 3000 | 5 W, replace each spring | Yes | In-pond all-in-one | 8.0 | about £119.99 | View offer |
On this page
Two greens
Pea soup is algae floating through the water. You can see it as a green cloud, and a sample in a jar looks green rather than clear with a few threads. Blanketweed is the hair-like growth on the liner, the plants, or the surface, sometimes in floating mats. The RHS page on pond algae and blanket weed separates those two. A UV clarifier is aimed at the cloud that passes through the lamp. It is not a treatment for the mat you can lift out with a stick.
If the water itself is green, you are asking whether a clarifier is the missing stage on a filter that already moves the pond. If the water is reasonably clear and the problem is strands, a new lamp will not comb them off the walls. The RHS lists a filter or a UVC as one option among others for algae, and it says wildlife ponds should be disturbed as little as possible. Barley straw appears on that RHS page as a separate suggestion. This page does not turn that mention into a dosing guide, and blanketweed treatments are a different category from these nine filters.
The same RHS filter guide says that if algae continues, the filter may be wrong for the fish, the feeding, the surface, the depth, or the sun, and that you should ask a specialist. A clarifier is not a substitute for an overstocked pond in full sun. Measuring the pond is the buying guide.
What the lamp does
The RHS account of a UVC is clumping. The lamp makes suspended algae stick together so the filter media can catch the clumps. It does not, on that account, sterilise the pond, and it does not scrape algae off the liner. The pad, the foam, or the bio-media still has to be in the circuit after the lamp. A lamp on a pond with no flow, or a flow that bypasses the media, leaves the clumps in the water for you to look at.
Some listings use a harder verb. The Blagdon bullet talks about the clarifier in the seller’s own wording, including the idea that it deals with green water. Prefer the RHS clumping line when you decide what you are buying, and treat the bullet as the seller’s claim. Bradshaws, a retailer rather than a lab, says a clarifier does not harm blanketweed or the algae on the walls, because those growths are not in the flow. It also says a bulb can still glow after the UV output has dropped, which is why a visible light is a poor test of whether the clarifier is still doing the job. Fish Mate’s instructions tell you not to look at the lamp anyway.
A few saved UK reviews on the Fish Mate 10000 and the 2500 GUV describe green water easing within about a week. That is those reviews, not a promise you can put on a calendar. The RHS does not give a day count. This comparison did not run a clearance trial, so a claim of clear in ten days is not one we will make.
Which of these nine actually say UV
Four rows mention a clarifier, and they do not mention it in the same place. The Blagdon Inpond 5-in-1 3000 states a 5 W UV on the bullet and says to replace the UVC lamp each spring. The pump is in the unit, titled to 3,000 litres at 700 litres per hour. Saved reviews still say the UV failed, died in weeks or months, or was missing. The Fish Mate 10000 puts combined UV and biological filtration on the bullets. Lamp watts are not in this Amazon file, and reviews say the indicator can be hard to see. The pump is not in the box, so the lamp does nothing until that pump is running water through the chamber.
The Fish Mate 2500 GUV is a gravity UV and bio filter with a quartzless chamber. The bullets describe the UV. They do not state lamp watts, and they do not include a pump. It is 4.5 from 710 reviews, and that star line does not cancel the reviews about bulbs that fail. The Fish Mate 15000 puts UV in the title only. The bullets describe filtration, Powerclenz, and SUPRA+, and they do not describe a clarifier. Saved UK reviews say it did not come with a UV and that the listing was not quite as described. If green water is the reason you are on that page, the title is the wrong line to trust. Which row matches the pond is the best pond filter page.
Five rows do not claim a UV lamp. The Biling is a pump, pads, and bio-media. The three POPOSOAP solar fountains describe filter boxes, panels, and, on the 8 W, a battery claim. None of those bullets names a lamp. The FEOXSHAL includes a pad and 20 bio-balls and says those prevent green water. Bio-balls are not a UV lamp. That sentence is the seller’s. It is not the RHS account of clumping. A solar fountain can move water in full sun. It does not become a clarifier because the water looks green and the box has a sponge.
Bulbs, windows, and a missing clarifier
A failed lamp removes the clarifier effect. The housing, the pump, and the foam can all be intact. Bradshaws advises a spring replacement, and the Blagdon bullet says the same: replace the UVC lamp annually in spring. That agreement is two sellers’ maintenance lines, not a measured life. Fish Mate reviews in this set describe lamps that failed sooner than a yearly habit. Do not turn a six-month or nine-month story into a guarantee, and do not keep a bulb that still glows if the water has gone green again and the rest of the filter is clean. Output and visible light are not the same thing on the retailer note. How to site the box, and why you do not look at the lamp, is the install guide.
If the title promised a clarifier and the carton has none, you are in a description problem, not a bulb problem. The 15000 is the row where that shows up in the saved reviews. Section 22 of the Consumer Rights Act 2015 gives a 30-day right to reject goods that do not match the description. This page does not promise that a particular refund will be approved. Photograph the title, the bullets, and the empty chamber while the window is open, and use the bullet list as the checklist.
The filter can still be the wrong size, the stock can still be heavy, and the pond can still sit in sun all afternoon. The RHS sends that remaining green to a specialist rather than to a second gadget. A new canister from a different brand, without a lamp in the bullets, repeats the gap you already have.
Common questions
Do I need a UV clarifier?
You need one if the pond is green with suspended algae and you want that algae clumped so the media can catch it. You do not need one, on the RHS split, to remove blanketweed or algae stuck to the walls. In this set the Blagdon states 5 W, the Fish Mate 10000 and the 2500 GUV put UV on the bullets, and the 15000 puts it in the title only. The Biling, the FEOXSHAL, and the three POPOSOAP rows do not state a lamp.
Why is the pond still green with a filter running?
The filter may be catching solids while the green is algae the pads never see as clumps. That happens when there is no UV, when the lamp has died, or when the pump is off for most of the day, as with a solar panel after sunset. It also happens when the problem is blanketweed rather than pea soup. The RHS adds stocking, feeding, surface, depth, and sun, and then a specialist if it continues.
Will a pond UV clear blanketweed?
No. Blanketweed is the hair-like growth, attached or floating as strands. A clarifier treats algae suspended in the water that passes the lamp. Bradshaws says it does not harm blanketweed or algae on the walls. Lifting strands out, and any treatment sold for them, is a different job from choosing a UV stage on these filters.
How long until green water clears?
There is no day count on the RHS filter page, and this comparison will not invent one. A few UK reviews of the Fish Mate 10000 and the 2500 GUV describe easing within about a week. That is an account from those buyers, and it assumes the lamp and the pump were working. A glowing bulb is not proof of UV output, on the retailer note from Bradshaws.
The UV bulb died. Is the whole filter broken?
The clarifier is the part that has stopped. The housing can still trap solids, and the pump can still move water. The Blagdon bullet tells you to replace the UVC lamp each spring, and reviews on the Fish Mate UV boxes describe failures sooner than that. Fit the lamp that model takes, then judge the water. Do not look into the chamber to check the glow. Fish Mate’s instructions say not to.
Do bio-balls replace a UV lamp?
No. Bio-balls and pads are media. They are not a clarifier. The FEOXSHAL bullet says the pad and bio-balls prevent green water, and the listing does not name a UV lamp. The RHS mechanism is a lamp that clumps suspended algae so the media can catch them. Media without that lamp does not become the lamp because a bullet says green water.
Does a solar pond filter clear green water?
Not as a UV clarifier. The POPOSOAP 4 W, 8 W, and 20 W do not state a UV lamp. They move water when the panel is lit, and the 4 W and 20 W do not state a battery. Suspended algae can keep multiplying through the hours the pump is stopped. If the pond is pea soup and you need clumping, you are looking at a mains unit whose bullets actually name a UV chamber.
Related pages
Sources
Public UK sources for this comparison. Listings and prices were taken from amazon.co.uk on 19 October 2026.
- RHS: Choosing a pond filter (retrieved 19 October 2026)
- Water Garden: pond filter selection guide (retrieved 19 October 2026)
- Water Garden: pond filters, including the OASE volume rule of thumb (retrieved 19 October 2026)
- Electrical Safety First: garden electrics (retrieved 19 October 2026)
- Electrical Equipment (Safety) Regulations 2016, regulation 35 (retrieved 19 October 2026)
- Consumer Rights Act 2015, section 22 (retrieved 19 October 2026)
- Approved Document P, 2013 (retrieved 19 October 2026)
- Fish Mate filter instructions (retrieved 19 October 2026)