How To Size and Use a UV Pond Filter To Stop Algae

A UV pond filter uses ultra violet light to kill e-coli and other bacteria as well as algae.

Who Really Needs a UV Light?

People who have a lot of fish in the pond can really use a uv light system to keep their water clean. High fish populations put extra pressure on water quality and uv-lights are a great way to combat this.
Understand there is a major difference between a fish pond and a natural pond. In a fish pond, the emphasis is on keeping a lot of fish – in the natural pond, it is keeping a few fish in balance with the size of the pond.

What it essentially does is sterilize the water in your pond.

And quite frankly, the use of or need for an uv pond filter signifies that there is a major problem in the design and/or function of the naturali> pond. Something is seriously out of whack.
In good pond construction, there is no need for this kind of filter system, the skimmer removes solids and the bio-filter deals with balancing the bacterial counts and changing bad guys to good guys .

Gardeners using ponds as fish ponds to grow and maintain large numbers of fish and not trying to create and maintain a natural pond with plants have a real need for a uv pond filter.

So if you want to sterilize your water with this kind of system, what are you looking for?

You want a UV pond filter system that will handle at least half of the flow rate of your pond pump every hour. This means if your pump is moving 500 gallons an hour, you want a pond filter that will handle 250 gallons an hour.
This table is a general guideline and is not meant to be the final word on all products. It is intended to give you a sense of how large your uv filter should be when compared to your water flow and pond size.


You want a system that is easy to maintain. If you can’t get it apart to change the light bulb (they do burn out more frequently than you might think – especially if outdoors) when it is on the store counter, it will not get easier when installed. This means you also want to install it where you can reach it easily.

String Algae

Your problem is not string algae. Interestingly enough, string algae is not usually killed off by these systems (although they are reduced).


You need to install the pond filter with a ground fault interrupter plug (gfi receptacle) to avoid electrical shocks.
The inlet and outlet of the filter should be as large as possible to eliminate the restriction and increased water pressure (both reduce the flow of water through the filter).
I hope this gives you some sense of uv pond filter systems and what to look for in purchasing one.

Sources for Products Mentioned on This Page

Pond UV lighting sources – for all sizes of ponds

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