Choosing the right Uv light for your pond post

It’s easy to get lost in the sea of available options when you are trying to choose the right UV light for your pond. Don’t get overwhelmed, let us help you make an informed decision. The right UV light for your pond is based on the size of your pond and the flow rate of your pump. Let's take a closer look so you know exactly what you need, and how to incorporate it into your pond design.

You can go straight to our top recommendations of UV lights based on pond size using the quick navigation below.

Recommended UV light

(Table of Contents)

Small pond

medium pond

large pond

UV lights do three major things for you and your pond.

  1. Kill harmful single celled organisms
  2. Improve water quality
  3. Destroy free floating algae

Neglecting to add a UV light into your filtration system is a rookie mistake. We know because we’ve made the same mistake early on. You worked hard to get your beautiful landscape. Having your crystal clear water turn to pea soup is frustrating.

Sizing your UV light before and after

green pond water as a result from an algae bloom
Green Pond Water From Free Floating Algae Bloom
clear pond water from a UV light
clear pond water After UV light Installation

What Is UV Light

Most of the UV light we encounter is from the sun. UV light (short for ultraviolet) is what is responsible for that awesome tan you get in the summer. It is also responsible for that nasty sunburn. 

UV light frequencies are electromagnetic radiation and have the power to break chemical bonds. This process is damaging to living tissue (sunburn).

Your pond UV light takes advantage of this by targeting the algae floating in the water. The radiation destroys the chemical bonds that make up the algae’s cell wall.

visible light spectrum

Light that we see is only a small fraction of the electromagnetic spectrum. The electromagnetic spectrum includes all forms of light. The light spectrum breaks down into 7 different categories based on wavelength.

Light Wavelength From Lowest To Highest Frequency

  • Radio
  • Microwave 
  • Infrared
  • Visible Light (All Light Detected By The Human Eye)
  • Ultraviolet (UV)
  • X-Ray
  • Gamma Rand Ay

The UV light frequency falls between what we can see as visible light and x-rays. When in contact with UV rays, atoms and molecules can become ionized.

An atom that's ionized loses electrons. The atom or molecule will have a negative or positive charge as a result. This is harmful to living tissue.

A UV light harnesses this property. Disinfecting surfaces, sterilizing water, and wiping out free floating algae in your pond.

How Is UV Light Used In A Pond

A UV light used in a pond consists of an outer tube with a smaller crystal tube running through its center. The smaller crystal tube contains the bulb that gives off the UV light frequency. Water flows through the outer tube, around the crystal tube exposing the water to the UV light rays.

How a UV light works to clear pond water

UV Light As A Clarifier

This is the best application for a pond or watergarden. It allows for a higher flow rate and lower wattage than its sterilizer counterpart. 

For this application the intensity of the UV light should be about 10 watts for every 1,000 gallons of pond water. The manufacturer of the UV light you choose will have suggested flow rates for both applications. 

UV light as a Sterilizer

Reduced flow rate, and the power of the UV light is increased to allow maximum exposure time. Sterilizers are normally used for aquarium applications.

To sterilize the water it is necessary to increase the strength of the UV light. You will need approximately 30 watts for every 1,000 gallons of pond water.

How To Size A UV Light For Your Pond

You need to know two things to get this right:

  • What is the size of your pond in gallons, use our pond calculator if you are not sure.
  • What is the effective flow rate of the UV light for your application (clarifier or sterilizer)?

Pond UV Light Recommendations Based On Pond Size

Here’s our recommendations based on pond size. If you aren’t sure what size your pond is, use our pond calculator. The UV lights listed in each category have a recommended flow rate for each of their applications (clarifier or sterilizer).

We are only recommending the lights below because it is what we would use in our pond builds.

*** Note: UV lights used as sterilizers are usually for saltwater aquarium applications. We list both specifications, a clarifier is what is used for ponds and water gardens

UV Light Choice  For Small Ponds

100 to 1500 Gallons

For this size pond we recommend the Aqua Ultraviolet Classic 15-Watt. With 15 watts of UV power, it will clear up your green water and keep it that way. Used as a clarifier it is rated to clear up to 2,000 gallons. 

Aqua UV light Classic-8-15-Watt white

We like to be a bit more conservative and suggest stepping up to the next size for anything over 1,500 gallons. This way you will be sure to clear the water effectively.

If you are using a pump that is 700 gph or less, this light will work for you.

Installing this light in line with your existing system is the easiest solution. If your flow happens to be higher than 700 gph valves can be installed to control your flow rate. See the video below.

Another option is to set this light up with its own pump. Doing so will not disturb your existing landscape. 

*Estimated operating cost per month $2.04

Manufacturer specifications:

Fresh Water Clarifier to 2000 gallons

Fresh Water Sterilizer 200-500 gals / Flowrate – 700 gph for 30,000 µw/cm2

Salt Water Sterilize to 75 gals / Flowrate – 233 gph for 90,000 µw/cm2

Aqua ultraviolet 8w-15w uv sterilizer clarifier black

UV Light  For Medium Pond

1500 to 3000 gallons

More water requires a larger light. This way there is more surface area for algae and parasites to be exposed. Step up in size to the 40 watt range for this size pond. This size can also be used for a small pond, but it is not necessary.

UV light features Aqua Ultraviolet Classic

Again we chose the Aqua brand. The Aqua classic 40 watt light is the perfect addition to your backyard oasis. No more chemicals, these lights are guaranteed to clear up your green water in 5 days or less when properly sized.

UV light Aqua Ultraviolet Classic 40 watt clarifier

This light claims to treat up to 6,000 gallons. We recommend stepping up to the next level for ponds over 3,000 gallons. This light will handle a flow rate up to 2,500 gph when used as a clarifier, and is what we would use in our builds. 

Check our pond shop for the best price on this light 

*estimated operating cost per month $9.05

Manufacturer specifications:

Fresh Water Clarifier to 6000 gals

Fresh Water Sterilizer 1200-2000 gals / Flow rate – 2900 gph for 30,000 µw/cm2

Salt Water Sterilizer to 325 gals / Flow rate – 967 gph for 90,000 µw/cm2

UV Light Choice Large Ponds

3000 To 5000 Gallons

When it comes to very large ponds more than one light may be necessary. Large ponds could mean anything so we set a maximum size at 5000 gallons. 

These lights have a large footprint and will take up some space. Take that into consideration when you are in the planning stage. Give yourself easy access for routine maintenance. You can camouflage your setup to disappear into the landscape.

UV light Aqua ultraviolet 25w-57w uv sterilizer black

Aqua UV is perfect for larger applications. It claims to handle up to 6,500 gallons as a clarifier. Keep your pond sparkling clear.

This UV light is available with or without a wiper. The wiper cleans the crystal tube that contains the light bulb without having to take it apart to clean. This keeps the light output high with very little maintenance.

The Aqua UV 57 watt UV sterilizer is guaranteed to clear your water within 3 to 5 days if properly sized. For large ponds this lights electricity consumption comes in at only 57 watts. 

Our pond shop has the best price on this light.

* estimated operating cost, only $12.90 per month.

Manufacturer specifications:

Fresh Water Clarifier to 6500 gals

Fresh Water Sterilizer 1500-3000 gals / Flowrate – 3200 gph for 30,000 µw/cm2

Salt Water Sterilizer to 355 gals / Flow rate – 1066 gph for 90,000 µw/cm2

*All operating costs in the descriptions above are based on a price of $0.31per kwh and calculated from

As you can see from our selections above we are a bit biased towards the Aqua brand. That’s because we believe they have the best UV lights around. Don’t just take our word for it. Give one a try in your pond. It will make a world of difference.

UV Light Maintenance

How Long Does A UV Bulb Last

The lifespan of a typical UV bulb is around 10,000 hours. The bulb will still be emitting visible light beyond that time. Its effectiveness will be diminished.

If you run your UV light all year round, the bulb should be replaced every 12 months. If you only run your UV light 4 months out of the year, the bulb will need to be replaced approximately every three years.

How Do I Know When To Change My UV Bulb

Unless it is completely burned out follow the above recommendation and change it after 10,000 hours. You could wait until your water begins to get cloudy with free floating algae, but who wants to go through that. 

Here is a helpful video about pond UV light maintenance:

Will A UV Light Solve All My Algae Problems

The short answer is no. The reason for this, a UV light can only eliminate what flows through it. It does a fine job at that. Yet string algae that forms on rocks and spillways will not pass through the light. You will need other strategies to get rid of string algae (link on how to do that).

UV light maintains clear pond water
koi pond with clear water from the use of a UV light
clear pond water from the use of UV light

The right sized UV light is only one piece of the puzzle. Ridding your pond of unsightly types of algae is an attack on multiple fronts. A UV light will give you crystal clear water. When used in conjunction with barley straw and algaefix you have a winning combination. 

UV light placement in a finished pond 

Now that you know what size UV light you will need it’s time to choose its location. When designed properly you will still have access to your return line. If not, it can be easily excavated and tapped into. Water can then be diverted through your UV light using control valves to change the flow rate. We discuss this in more detail in our UV light installation article.

Another solution would be to place a pump in your pond strictly designated to your UV light. This type of setup would allow you to size the pump to achieve proper flow rate for your UV light.

Either way your light can be hidden (even if it is large) with creative plantings. Check out this quick video.

Disguising your UV light in plain sight:

When you install your light be sure to give yourself easy access to it. You will need to do occasional maintenance. If you live in a cooler climate, your light will need to be disconnected for the winter.

It’s a good idea to have your UV light on its own “water circuit”. If you plan on running your circulation system all year round, being able to turn off the flow to the light and remove it is crucial in cooler climates.

This can be accomplished by diverting water that flows from your skimmer to your waterfall with a few PVC valves. This also allows you to adjust your flow rate to whatever you desire. Check out the video below on how to do this.

Installing a UV light in an existing water feature:


Choosing the right UV light for your pond will make a huge difference in the enjoyment you get all season. The constant battle with algae can be very frustrating. If you are getting started, take my advice and don’t overlook the importance of installing a UV light.

Yes, you can always add one later. It’s definitely easier if you make it part of your original design when you build your pond (link to our FREE Ultimate How to Build a Pond Guide).

You can’t go wrong with any of the suggested lights in this post. All of them are available in our pond shop. Now you know exactly what you need to get your clear water back and keep it that way. Don’t wait until next season, get your light now and get maximum enjoyment from your backyard oasis. Until next time, enjoy your backyard water garden

About the author


I love being outdoors and hands on DIY. If you share my passion, you are in the right place. If I can help one person surprise themselves with the success of a project, then this website is a success.

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  1. I simply couldn’t go away your site before suggesting
    that I actually enjoyed the usual information a person supply to your guests?
    Is gonna be again continuously to check up on new posts

  2. Ok. I have a newly constructed 4800 gal pond. I’m running an aqua scape 3500 pond filter with a UV light. I don’t think it’s enough of a light. You suggest a second light. Which light and would you run it off the pond filter on the outflow line back into the pond? The skimmer runs a big pump to a waterfall about 100 feet away which goes into a stream bed; that seems a bit long for a power supply line. At least the pond filter is near the pond and electricity.

    1. From what I see the 3500 only has a 25watt UV light. It also looks like it is an “all in one system”.

      No this is not a powerful enough light to keep your pond clear. You could add an additional light somewhere along the return line. An additional 40 watt light will be enough to handle your 4800 gallon pond.

      By adding it to your existing return line it should be a relatively easy job. Here is a link to a video I made installing a new UV light to an existing system.


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