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How Do UV Pond Lights Work?

A well-maintained pond is a great feature in your garden, especially if you have fish.

It’s picturesque and can be pretty soothing.

The actual process of maintaining a pool is something you have to work to get right – they don’t stay clean and clear on their own.

A murky pond won’t look near as pleasant – plus, if you do have fish, you won’t be able to see them.

Which is when a UV pond light may be beneficial.

UV pond lights kill single-cell algae that can cause water to turn green and murky, helping to keep the water clear. They are not a filter or kill string algae, but they are good to use alongside other maintenance tools.

To explain whether it’s worth buying a UV pond light, let’s take a look at:

  • The purpose of UV pond lights
  • How long they take to clear a pond
  • How long UV pond lights last
  • Whether UV light is good for Koi ponds

What Is The Purpose Of UV Pond Light?

pond with stones in the backayard

The purpose of a UV pond light (Amazon) is to help clear the water in your pond.

Pond water, if left untreated, will naturally turn murky due to the growth of algae within the pond.

There are different types of algae, but the single-cell algae floats within the water, which causes the murkiness.

A UV pond light kills these algae, destroying the DNA. As it is destroyed, the water becomes clearer, so you can see through it.

UV pond lights only work on the water as it passes by, so you need the pond water to move.

Also, it is not a filter, although some may be falsely named a “UV pond filter.”

It doesn’t treat the water for anything other than the single-cell algae, killing that (and sometimes other microorganisms in the pond – more on that later).

Also, don’t confuse clear water with clean water.

Many people assume that a clear pond must be a sign of good quality water and, therefore, good for fish, but there are a lot of other parameters that must be checked.

How Long Does It Take For A UV Light To Clear A Pond?

how long does it take to clean the pond

There are a couple of different factors that will determine how long it takes for your pond to be cleared.

One of those is making sure you have the right size of UV light for your pond. You’ll need a UV light with around 8-12 watts for every 3,000 gallons or 10,000 liters.

If you have the right size for your pond, you must ensure the water is circulating past the light.

It should constantly move slowly using a pump so that every bit of water gets treated.

When pond water is stagnant, then the UV light won’t work.

If the pump is too powerful, the UV light may not be able to kill the algae as it circulates past.

So, provided you have the right size of UV light and your pump is working at an appropriate speed, you should notice a difference in around 3-5 days.

Larger ponds may take up to two weeks at most to clear.

How Do I Know If My Pond Light Is Working?

private pond in evening

Sometimes it can be difficult to know if a UV pond light is working.

They are often submerged where you can’t see them, and if you bring them out of the water, they may have a trigger to switch them off automatically.

That’s because it’s not safe to look directly at a full glowing UV lamp.

As a safety feature, those triggers prevent the light from working with an exposed bulb.

However, you should still be able to see them underwater because they typically have a small safe window in the bulb.

This is designed to let you see that it is working without looking directly at the whole glowing bulb.

If you can’t see it working, there might be a problem with the circuitry or the ballast, or it could just be time to replace the bulb if it’s burnt out.

Does UV Light Kill Beneficial Bacteria In Pond?

Beautiful classical design garden fish pond

While researching pond bacteria online, you’ll see mixed messages about whether UV lights also kill beneficial bacteria.

Some people say that they do. Others say that they don’t.

Firstly, always be wary of what they’re trying to sell you!

A UV pond light manufacturer will downplay concerns, while someone selling a ‘bacteria balancer’ will tell you how UV pond lights aren’t a good idea.

The truth is that UV pond lights kill some good and bad bacteria.

But usually, there are enough good bacteria left in a pond to balance things out.

If you’re concerned about the balance of bacteria in your pond, you can add a secondary sand filter.

These are good breeding grounds for beneficial bacteria. So if you install one away from the UV filter, you should have a healthy water balance.

Adding rocks to the bottom of the pond can also help.

How Long Do Pond UV Lights Last?

beautiful landscaping with beautiful plants

The typical lifespan of a UV pond light is around 10,000 hours.

Sometimes you might see the bulb still working after this, but you still need to replace it even if it hasn’t burned out – the UV levels will diminish, making it a lot less effective.

If you leave the UV light 24 hours per day, you’ll burn through 8,760 hours within a year.

So, it’s recommended that you replace your pond UV lights every 12-18 months.

Can I Leave The UV Light Always On?

Beautiful Garden With A Little Pond

It’s recommended that you leave your UV pond lights switched on at all times, although some people prefer to switch them off and remove them in the winter.

This is because the lower temperatures don’t encourage algae growth but can potentially damage the UV light if you live somewhere with particularly harsh winters.

Turning a UV light on and off regularly will lower its lifespan, so it isn’t recommended to have one on a timer.

They won’t harm fish, so it is best to leave them switched on.

You should switch them off only if you need to add a bacteria balancer product to your pond or if your fish are given medication that stipulates no UV light.

If you’re adding a bacteria balancer, switch off the UV light for two days to give the product time to establish itself.

For medication, leave the light off for the duration of the medication you’re giving to your fish.

Is UV Light Beneficial For Koi Ponds?

Red gold and white koi fish in a pond

UV light is beneficial for Koi ponds, but no more so than with other types of ponds.

Koi are sensitive fish, but other water parameters such as acidity, temperature, and oxygen levels are more important.

Single-cell algae won’t really affect these, but UV pond lights won’t harm Koi, provided you use other filters to help encourage beneficial bacteria to develop.

Of course, with Koi being so expensive, you’ll want to see them – so a UV pond light benefits Koi owners who want to enjoy clear water!

But the more visible your Koi is to you, the more visible they are to predators, so make sure you provide hiding spots for your fish.

Final Words

UV pond lights may be costly, but they’re beneficial for keeping your pond clear of single-cell algae, making it murky.

If you don’t already have UV pond lights installed, they’re worth considering if your pond gets that green, murky look.

And if you inherit a pond with a new home, check the UV lights are working – it may be worth replacing them since even if they glow, they might not have the necessary UV strength.

Have you used a UV pond light to clear your own pond? How much better did it look after treatment?