Pond UV lamp: when to turn it on and off throughout the season

A UV-C lamp destroys suspended green algae and keeps the water clear. But it does not need to run all year round, and there are times when it must be switched off. Here is the simple calendar for a pond UV lamp.

In spring: restarting

Algae start growing as soon as the water warms up and the days get longer. Switch on the UV when the water reaches around 10 to 12 °C. This is the right time to change the bulb: a UV-C bulb loses most of its effectiveness after one season, even if it still lights up.

• Clean the quartz tube: limescale or biofilm deposits block UV rays.
• Check the seals.
• Check that the indicator light comes on properly.

In summer: 24 hours a day

It's green water season. Leave the UV running continuously, at the same time as the pump. A UV that runs only a few hours a day lets algae multiply the rest of the time.

During treatment: switch it off

UV-C rays also destroy treatment products and some of the added bacteria. Switch off the UV:

• during a treatment, according to the product instructions;
• a few days after adding bacteria, while they settle in the filter.

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Regarding bacteria, read Should you switch off the UV when adding bacteria to a pond?.

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In winter: switch it off and protect it

Below 8 to 10 °C, algae hardly develop anymore. A UV unit that freezes may have its quartz shatter. Turn it off, drain it, and bring it indoors if the manufacturer recommends it.

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To check whether your UV is still working, see How can I tell if my UV is still working in my pond?.

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Frequently asked questions

— Is the UV lamp dangerous for fish?

No. The UV rays remain enclosed inside the device, and the water passes through without exposing the fish. Never look at a lit bulb outside its housing.

— Does UV kill filamentous algae?

No. It acts on algae suspended in the water, not on those attached to walls or stones.

— My water remains green despite the UV, why?

Bulb too old, quartz dirty, flow rate too high, or insufficient power for the volume. Check these points in this order.

In summary

• New bulb and clean quartz in spring.
• 24 hours a day in summer.
• Switch off during treatments and after adding bacteria.
• Shutdown and frost protection in winter.

Last updated: September 27, 2026.