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What you need to know about UV filters for ponds

What you need to know about UV filters for ponds
Does your pond look like pea soup?

So, your pond has the “pea soup syndrome”? And you have finally decided to do something against the algae crowding it?

Or, maybe you want to reduce the risk of disease pathogens in your pond, to avoid trouble with your fish and plants?

In both cases a UV pond filter is a superb solution to your problem. This article outlines all the basics and essentials you need to know.


Learn about:

– how they work
– where to place them
– whether you need other equipment with it
– and what you should absolutely be aware of before buying one

Let’s get started:


How do UV filters for ponds work?

When water is pumped from your pond through the UV filter, the single-celled algae are exposed to very high levels of ultra violet light. This destroys their DNA and causes the algae to divide. And when they divide, they die, resulting in them clumping together.

These clumps of dead algae can then be removed from the water.

A UV pond filter is NOT a ‘filter’

Technically, a ‘uv filter’ is not a filter, because it doesn’t physically remove the dead clumps from the pond.  It only kills the free floating algae, and is usually called a ‘clarifier’ or a ‘sterilizer’.  You still need a filter along with it. So from now off on in this article, we’ll be referring to these UV pond filters as “UV pond lights”.

What kind of filter do you need to get rid of the dead algae?

There are two options:

  1. The simplest: you can use a mechanical filter to get the dead matter out. Well, rather, YOU get the stuff out by backwashing and cleaning the filter.
    In the beginning when you start using the uv light in the pond you have to regularly do this. When the water becomes less green, there will be less algae to clear out and thus the filter will remain a lot cleaner.
  2. You can also use a biological filter. Instead of physically removing the dead algae and other biomatter from the water, these are trapped in a tank that contains beneficial bacteria. And it’s these ‘good’ bacteria that decompose the organic waste materials.

For more on the difference between a mechanical filter and a biological filter see this article (coming soon).


Where do you place the UV light?

The obvious: The uv light is placed inline with the pump and the filter (either a mechanical or biological filter). The pump gets the pond water to the uv clarifier/sterilizer, and the filter takes care of/removes the dead algae.

The not-so-obvious: Do you place the uv light before or after the (mechanical or a biological) filter?

Generally, the better, and most common, placement is right after the filter. In fact, a lot of the biological filters (that have a uv light built in) place this uv light after the filter itself. There are a couple of good reasons for this:

  1. The uv light works more efficiently when the water flowing along it is clearer. When there are more particles and debris filtered out, the light can penetrate deeper into the water, and reach more algae.
  2. Any small rocks being sucked up by the pump and transported to the uv light can harm the quartz sleeve around the bulb. Having a filter before the light prevents damage to the quartz sleeve.
  3. A filter before the light traps more sludge, thus slowing down the build up of sludge in the uv light casing. It creates more sludge in the filter itself, but you need to clean that regularly anyway.
  4. Ultraviolet light also kills ‘good’ bacteria. Usually this is not something to worry about, because the bacteria tend to live along the sides of the pool and on the bottom. But if some are free-floating, it helps having the filter before the uv light: the good bacteria might stick to the filter material (the pad) itself.

show picture of replacement pond filter pad

To learn more about the “before-filter-vs-after-filter” question in more detail, check out this article: Should you place the uv pond light before or after the filter?


Another thing to keep in mind about the placement of the uv light

Put the pump that feeds the uv light far away from the tube that returns the water to the pond.

If you place it near the return (or any other outlets such as waterfalls, foutains or aerators), you just pump up relatively clean water, thus decreasing the effectiveness of the whole inline filter system. The algae in the water in some parts of the pond will hardly be affected in that way. So prevent this and put it far away.


More essential knowledge about uv pond lights

You need to clean

Obviously, you need to clean the (mechanical or biological) filter from the waste, debris and dead algae. This is especially true when you have just installed and activated the uv light in your pond, because of the die-off of the initial excessive amount of algae. This will get better over time as your pond gets clearer, up to a point where you only will need to clean the filter as a regular maintenance.

Apart from this, you will also need to clean the actual uv lamp.

As said, it is advisable to have a filter before the uv light, to prevent large particles and dirt to accumulate inside the UV light casing. But even with this precaution, you will have some build up of dirt and calcium on the quartz sleeve. And that reduces its effectiveness. So, it’s a good idea to clean this every year or so. One word of caution: when you do this, be gentle, as the quartz tends to become more brittle over time and there’s a risk of damaging the sleeve.


UV lights don’t work with hair algae

UV lights kills algae because water, with the free floating algae in it, passes over the uv lamp. Hair algae, which are algae that form strings and are attached to the bottom and the sides of the pond, obviously aren’t free floating and thus do not pass in front of the lamp and are not affected.


Are they submersible?

Some of these uv lights are submersible, and can be used under water or out of the pond. Most are not submersible. It depends on the model.

The above is true for the standalone types of uv lights that you have to place inline with your pump and filter. However, most, if not all, of the biological filtration systems that include a UV light, are meant to be situated outside of the pond.

There isn’t really an advantage to having a submersible model vs a out-of-pond model. Maybe aside from the fact that (with the submersible types) you have to attach a weight to the light if you don’t want it floating on the surface of the water. But I doubt this can be called a real disadvantage.


The slower the flow, the better

If the water flows by too quickly the uv light doesn’t have a chance irradiate all the algae.
The slower it goes the more efficient the process. That’s why these uv light filters all have a ‘max flow rate’ specification.


If UV light kills microorganisms, is it dangerous to us as well?

Under normal use, they’re not. But yes, prolonged exposure of the light to the skin can be harmful. And staring into a UV light is dangerous and can cause eye damage.

That’s why wearing sunglasses is a good idea when handling the naked (and lit) bulb.

However, there might be no need to do this when the uv filter has a “viewing window”. If this is the case, simply only turn on the light when it’s fitted in its casing. Even a clear or transparent glass or plastic viewing window will block the majority uv radiation reaching your eyes. All you will be seeing is a blue light.

Actually: that’s why the sleeve around the lamp is made of ‘quartz’. Quartz is a material that doesn’t block the uv, thus allowing it to penetrate the water around the sleeve.

My suggestion is to avoid looking at the light directly (even through the viewing window), avoid spending a lot of time looking for the blue light during the day (when it’s harder to see the light), and even use sunglasses to be on the safe side.


What purpose does the quartz sleeve serve?

Apart from making sure no water enters the electrical parts of the lamp, it also does a good job at providing a protective barrier around it, so that it can operate at its optimal temperature (the ideal temperature for the lamp to operate is 104 Degrees Fahrenheit -or 40°C).


Do you run the uv light 24/7, or do you turn it on and off?

There are many people who turn it off and on as required.
However, in ponds with a lot of fish, few plants and a lot of strong sunlight it might be wise to leave it on all the time in order for the uv sterilizer to ‘catch up’ with new algae growth. It all depends on the size of your pond and circumstances.

For example, you might want to choose to have it on full time in the beginning, and eventually put it on only 6 hours a day during the down season.


Do I need to replace the bulb once in a while?

Yes, a uv bulb is said to lose its efficiency after about 9000 hrs. of use. Practically this means that they should be replaced yearly. The guideline is to replace your bulb at the beginning of the season, using them continuously over the next 9 months (or however long your season is).

After one year, they might still produce visible light, but it most likely will have stopped producing the UV wavelength, and they will not be doing what they’re supposed to be doing anymore. They gradually decrease in effectiveness. So replacing the bulb every year, will improve performance. (Note: a typical price for a new bulb is around $100.)



So, there you have the basics you absolutely need to know when acquiring a UV pond light. This article should have given you a good idea what uv filters in ponds are all about, how they work, the things to be aware of before installing them and how to treat them.