A pond UV is needed to help clear algae from your pond. Using the correct size UV on your pond is essential. When choosing a UV for you pond it is recommended that you select a unit that has a pond size recommendation a little over the total water volume of you pond, this will ensure that 100% of you water within you pond will be effectively exposed to the ultra violet light that the UV emits. The most efficient setup up for you pond will depend on the current filtration you have, in most cases an inline UV can be used within the feed pipe from you pump to you filtration system.
UV lights can be purchased as standalone units or complete set ups with the filter included, most pressurised and all in one filters have a UV built in.

Important: Disconnect mains electricity supply before handling or attempting maintenance. Always fully read and check the troubleshooting section within your product instruction manual. 
So you already have a UV Steriliser, when was the last time you changed the bulb?
Regular maintenance on you UV system will keep algae/ water clarity problems at bay, an old bulb could be the reason behind why algae is becoming problematic, by making sure the quartz sleeve is clean and the UV bulb is replaced regularly (every 6-8 months), will ensure that the water passing through your UV unit is eradicated of the unwanted pathogens and algae spores.

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If you have recently installed a UV into you system/ pond, but there does not appear to be an improvement to you water clarity, checking that the UV bulb is functioning should be your first step.

If you bulb is functional, then the next step would be to perform regular small water changes. (Around 30% of the ponds volume)

NOTE: When changing water within you pond please make a note of the amount of water you are removing, this is because when you add the water back to your pond it is recommended that you treat the new water with a De-chlorinator to remove the harmful chlorines that are used to treat tap water for human consumption. These chlorines can irritate the gills of your fish and also kill the beneficial bacteria that reside within you Pond filtration system.

TIP: Please keep in mind that the UV is a preventative not a cure to an existing problem. This means that should you pond already be “green (pea soup)” then it may take many weeks for the UV to catch up and clear the water. The solution for this is before installing the UV unit (or switching the UV unit on for the first time) complete a 2-3 30% water changes (following the steps above) and then treat with an anti-algae treatment, this will help to remove the algae that is within the pond and allow the UV unit to prevent reoccurrence.

Find out more about how a UV steriliser can help combat algae.