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Pond UV Clarifiers: What to Look For

A UV clarifier clears green water by treating free-floating algae as it passes the lamp. Flow rate decides whether it works.

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Quick answer

Choose a UV clarifier rated for your pond volume and keep the pump flow at or below the unit’s stated maximum, so the water spends long enough in front of the lamp.

A UV clarifier exposes water to ultraviolet light, which causes the single-celled algae responsible for green water to clump together so the filter can remove them. It does not affect blanket weed or string algae growing on surfaces, and it does not replace a filter.

Because it depends on exposure time, a UV clarifier is sensitive to flow. Push water through too fast and it becomes far less effective.

What to compare

  • Rated pond volume. As with filters, use the rating for a pond with fish and in sun if that describes yours.
  • Maximum flow. This is the number to compare with your pump flow at head.
  • Lamp life and replacement cost. Output fades with use even though the lamp still glows. Manufacturers typically specify a replacement interval.
  • Inlet and outlet sizes. Narrow connections add head and restrict flow.
  • Position in the system. A separate UV unit is normally fitted after the pump and before the biological filter. Follow the manual.

Where this type works, and where it does not

A good fit when

  • Green, pea-soup water caused by suspended algae
  • A pond in full sun
  • You already have a working filter to remove the clumped algae

Look elsewhere when

  • The problem is blanket weed or string algae
  • There is no filter to catch what the UV treats
  • Your pump flow far exceeds the UV unit’s maximum

Compare current options

These links open live Amazon search results rather than a fixed list, so you always see what is currently available. Check the flow at your own head height, the warranty and the cord length on the listing before you buy.

Common questions

Why is my pond still green with a UV clarifier?

The usual causes are an old lamp, a dirty quartz sleeve, or flow that is too high for the unit. Check the lamp age and compare your pump flow with the clarifier’s maximum.

Is a UV lamp safe to look at?

No. Never look at a lit UV lamp outside its housing, and always disconnect the power before opening the unit. Follow the manufacturer’s instructions for lamp changes.

Electrical safety

Pond equipment runs on mains power next to water. Use outdoor-rated equipment on a GFCI (RCD) protected supply, follow the manufacturer’s manual and your local electrical code, and use a qualified electrician for any fixed wiring.