What size pond UV clarifier do I need?

A UV clarifier is for pea-soup water: the single-celled algae floating in the pond. It is a poor tool for the long green strands stuck to the liner. That difference is the whole buying decision, and it is set out on the green water and blanketweed pages.

What the lamp does

The RHS says an ultraviolet clarifier makes algae clump so the filter can remove them. It does not, by itself, strain the water. Hozelock describes the same path: water passes the lamp, the cells clump, and the filter catches them or they sink. Hozelock also says you may see a change in a few days, and that clear water can take six to eight weeks, depending on the pond. That is the maker's claim, not a promise.

The lamp only sees water that actually flows through it. Blanketweed anchored to the walls often never makes the trip. Hozelock's booklet also says some treatments are broken down quickly by UV, so switch the unit off during a disease treatment. That is a fish-health decision. If fish are ill, ask a vet or an aquatic specialist rather than dosing from a forum.

A published UK range

On 13 November 2026 the Hozelock Vorton page listed four bulbs, four maximum pond sizes and four recommended flows, in this order:

• 11 W, ponds up to 4,000 litres, recommended flow 2,000 L/h
• 18 W, ponds up to 6,000 litres, recommended flow 3,000 L/h
• 36 W, ponds up to 12,000 litres, recommended flow 6,000 L/h
• 55 W, ponds up to 18,500 litres, recommended flow 9,250 L/h

The page presents those as the range. Check the model you buy against the current chart, because a "maximum pond" with no fish is often twice the size of the same unit with fish. The Vorton booklet's older performance table has used the larger, unstocked figures. The product page figures above are the ones to start from for a fish pond, and the planner uses them that way.

The booklet also says that for best results the whole pond should pass through the clarifier every 1½ to 2½ hours. A 6,000 litre pond would want roughly 2,400 to 4,000 litres per hour. The 18 W model's recommended 3,000 L/h sits in that window. If your filter pump is far stronger than the clarifier's recommended flow, the water can race past the lamp. Throttle it or bypass it the way the maker shows. Do not assume a bigger pump makes a UV work better.

Above 18,500 litres, this range has run out. Oase's Bitron Eco, for example, is a different class of machine: the 180 W model is listed for up to 180,000 litres with no fish, 90,000 litres with fish, and 45,000 litres with koi, at flows from 10,000 to 50,000 L/h. Quote the line that matches your fish. Do not scale a garden-pond watt by guesswork up to a lake.

Koi are the awkward case. Oase's koi line on that large clarifier is about half the fish volume, which means more watts per litre. Hozelock's page does not print a separate koi column. If you keep koi, use a clarifier the maker rates for koi at your volume, not the goldfish number from a different brand.

One model, as listed

We haven't tested this. Figures are as the maker states them (checked 13 November 2026).

Hozelock Vorton 18 (model 1554). The Amazon UK listing (ASIN B007ENJ3Q8) says it suits ponds up to 6,000 litres, with a maximum flow of 3,000 L/h and an 18 W lamp. That matches the middle row of the Hozelock page. On 13 November 2026 the listing price was £120.74. The listing's model numbers are not perfectly consistent with each other (the title says 1554, and the detail table also shows 1554B1240 and 1554C1240). Match the spare lamp and the quartz sleeve to the version you have. Hozelock sells them as different parts.

View the Vorton 18 on Amazon UK

Replace the lamp when the maker says, not when it looks dark. Output falls before a lamp fails completely. The unit also needs the quartz sleeve clean. A dirty sleeve is a common reason a clarifier "stops working" while the bulb is still lit.

The clarifier is an electrical device on a pond. It belongs on an RCD-protected supply. See pond electrics.

Will a UV clear blanketweed?

Not reliably. Blanketweed is attached, so it may never pass the lamp. Pull the strands out, and read the blanketweed page before you buy a bigger bulb.

How long until the water is clear?

Hozelock says a few days for a visible change and often six to eight weeks for a full result. The RHS does not offer a day count. If nothing has changed after that sort of period, check the flow, the sleeve and whether the filter is actually catching the clumped algae.