Using an Ultraviolet Filter

Using an Ultraviolet Filter
There’s one type of filter that isn't often discussed but can help clear foul-looking water while keeping your fish healthy. It’s called an “ultraviolet clarifying filter” or UV filter for short. It’s basically an in-line sealed box containing an ultraviolet light fixture (in its own waterproof compartment). As water passes through one end of the box, past the ultraviolet light, and out the other end, the light kills off algae, parasites, and other harmful pathogens.

Of several types of UV filters on the market, the most effective ones use a quartz filter. The intense light that a UV filter generates penetrates the walls of the algae, or the pathogens, causing them to die off and clump together. The clumps are then removed as they pass through a mechanical filter.

The main drawback to using a UV filter is that it needs to run continually to be effective, so a light bulb normally lasts for no more than a year or so. You should change the bulb and clean the quartz filter annually, ideally in spring before algae has a chance to take hold, even if the light is still working, since the effectiveness of the UV rays diminish after that.

One thing about UV filters and algae: they don’t get rid of all forms of algae, such as the string types that cling to the rocks and sides of your pond. They only get rid of the suspended algae. But, since that’s the algae that makes water appear green and dirty, killing off the suspended algae will help keep your water clear and clean.

Since UV filters also kill off harmful bacteria, fungus, and parasites, they're particularly beneficial to fish keepers. Don’t believe me? Since adding a UV filter to our largest pond, our water has never been clearer, and we haven't lost a fish in more than four years!

Selecting a Filter System
So, what’s the best filtration system for your pond? I suggest a combination of different filter types—all four, if that’s possible. As we’ve seen, a single filter type simply can’t accomplish everything you want. After all, if you had a sick child for whom a doctor prescribed three different medications, you wouldn’t cut corners and give him only one, would you?

By combining mechanical, chemical, biological, and ultraviolet filtration methods, you’ll be creating the cleanest, purest, healthiest pond possible. Mixing filtration types insures a well balanced, well functioning pond. And, a well balanced, well functioning pond produces healthy, happy fish.

Check out D. J. Herda's two latest gardening books, Zen & the Art of Pond Building and From Container to Kitchen: Growing Fruits and Vegetables in Pots, both available from

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