why you need a UV Light in your pond

Why You Need A UV Light In Your Pond

Why Do I Need A UV Light In My Pond

UV lights do a few things for you and your pond. They kill harmful bacteria and sterilize the water, creating high water quality so goldfish and koi can thrive. The other thing… they destroy the free floating algae that plagues pond owners. 

Don’t neglect to add one of these into your filtration system. I know how frustrating it can be to create a beautiful pond, waterfall, surrounded by plantings only to have it turn into pea soup once the temperature gets warm. 

Green Pond Water From Free Floating Algae Bloom
clear pond water from a UV light
UV Light Installed – Clear Water

What Is UV Light

UV light (short for ultraviolet) is what is responsible for that awesome tan you get in the summer. It is also the culprit of painful sunburn. This type of electromagnetic radiation has the power to break chemical bonds and is damaging to living tissue (sunburn). Most of the UV light we encounter is from the sun.

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 spectrum is broken down into 7 different categories based on wavelength. These categories from lowest to highest frequency are radio, microwave, Infrared, visible light (all light detected by the human eye), ultraviolet (UV), X-ray, and gamma rays.

UV lights 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. 

When atoms get ionized they lose electrons which result in the atom or molecule having a negative or positive charge. This is harmful to living tissue. Because of this property, we can harness the power of UV light to disinfect surfaces, sterilize things, and wipe out all that nasty free floating algae in our ponds.

How Is UV Light Used In A Pond

A UV light for a pond consists of an outer tube with a smaller crystal tube inside it. The smaller crystal tube contains the bulb that gives off the UV light. The crystal tube keeps the bulb dry. Water flows through the outer tube, around the crystal tube, safely exposing the water to UV light. 

How a UV light works

How To Size A UV Light For Your Pond

In order to get this right you need to know a couple things first: 

  1. What is the size of your pond in gallons (use our pond calculator)
  2. What is the maximum flow rate the light can handle to be effective. Running 4000 gallons per hour (gph) through a light that can only be effective at a flow rate of 1000 gph is pointless and will yield poor results.

Where To Put My Light If My Pond Is Already Finished

This was my problem as well. My pond was built and the UV light installation was an afterthought. If you don’t plan on using a submersible UV light and wish to put it in line with your water return system, you can still easily hide your light (even if it is large) with creative plantings. Check out this quick video.

That’s what I did. My UV light is hidden in plain sight, tucked around the side of my waterfall mound. When you install your light be sure to give yourself easy access to it for occasional maintenance.

Which UV Light Is Right For Me

If you are just starting out building your pond (the best time to add a light) then you can easily incorporate this into your filtration system. Simply divert water that flows from your skimmer to your waterfall unit through your UV light.

If you are adding a light as a second thought it is going to be a little more involved, but there is still hope! You will have to dig up your return line in order to divert water through your UV light as it returns to your waterfall. 

Another option is to add an additional pump (usually with a lower flow rate to be effective) and add your UV light to that exclusively. 

If you plan on running your system all year round having your light on its own “water circuit” is a good idea. This way you can turn it off or remove it over the months it isn’t needed.

How Long Does A UV Bulb Last

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

If you live somewhere that 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 per year like I do, then the bulb will need to be replaced approximately every three years.

How Do I Know When To Change My UV Bulb

With a 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.

How Much Does It Cost To Run A UV Light For My Pond


This will depend on the size of the light that is required for your system, and the price of electricity at the time. If we were to choose a 40 watt light and the price of electricity is $0.31 per Kwh then the cost would be $9.05 per month based on the calculations from https://www.blitzresults.com/en/electricity/ 

Where Is The Best Place To Buy A UV Light

Here are some great options based on pond size. If you aren’t sure what size your pond is, use our pond calculator. I have found the best place to get any of these lights is on Amazon. Yes, these are affiliate links which means if you happen to purchase one of them I will receive a small commission at no charge to you. This helps us keep our website going. Thanks in advance for the support.

That being said we are only recommending these because it is what we would use in our pond builds.

Small Pond – 100 to 1500 Gallons

The Tetrapond UV clarifier is the ideal addition to a smaller pond. With a 9 watt UV bulb it will have little impact on your electric bill. This light can be used in or out of the water, making it an easy addition if you already have your pond set up.

If installing it in line with your existing system be sure the flow rate is between 210 – 560 gph. This light is most effective at this flow rate.

The other option is to hook it up with its own pump. Whichever is easiest. The recommended pump and light are bundled together in the link below. 

This light can handle a flow rate of 210 – 560 gallons per hour. Using a ¾ – 1 inch ID tubing to connect a pump and return line. 

Although this light claims to treat up to 2,000 gallons we are more conservitive and recommend using a more powerful light if your pond is over 1,500 gallons. 

This will solve your green water problems at a budget friendly price. Check HERE for the best current price (link to amazon) along with other helpful diagrams.

Estimated operating cost per month $2.04

Medium Pond 1500 to 3000 gallons

Stepping up to the next size will require a little more power and a larger unit. The 40 watt range is effective with a pond of this size. 

The stainless steel Patriot is our light of choice for ponds in this size range. The stainless steel is reflective making the UV light up to 35% more effective than other non reflective lights. 

patriot pond 40 Watt UV light specs

Although 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. This is what we would use in our builds. Check HERE (link to Amazon) for the most up to date price.

The estimated operating cost per month $9.05

Large Pond 3000 To 5000 Gallons

When it comes to very large ponds more than one light may be required, so we set the maximum size at 5000 gallons. The UV lights used for larger ponds are not submersible and will take up some space so be sure to take that into consideration when you are in the planning stage. You should give yourself easy access while at the same time camouflage your setup to disappear into the landscape.

Aqua UV is perfect for larger applications. It claims to handle up to 6,500 gallons as a clarifier. You will get your pond back. This light is effective with a flow rate of 3,500 gph which means you won’t have to sacrifice power when you run it in line with your system. It is available with or without a wiper. 

Aqua UV 57 watt UV sterilizer

The wiper cleans the crystal tube that contains the light bulb making it more efficient.

The performance of this light is guaranteed to clear your water within 3 to 5 days. 

Another advantage this has over other lights in this category is it’s electricity consumption, at only 57 watts. For the most up to date price and availability check HERE (affiliate link to Amazon).

The estimated operating cost, only $12.90 per month.

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. For that It does a fine job. However other forms of algae like string algae that forms on rocks and spillways also plague the pond. This type of algae will not pass through the light because it stays in one place. 

If your pond is overrun with string algae check out our home page and learn the secret to give it a quick knockout punch. This little secret is safe, inexpensive and eliminates string algae quickly.

A properly sized UV light is only one piece of the puzzle to ridding your pond of unsightly types of algae. A UV light will give you crystal clear water. When used in conjunction with barley straw and algaefix you have a winning combination.

In Conclusion

If you have had a pond for any length of time you know the value of a UV light. The constant battle with algae can be very frustrating. If you are just getting started, take my advice and don’t overlook the importance of having a UV light. Yes, you can always add one later, but it is definitely easier if you make it part of your original design when you build your pond (link to our how to build a pond tutorial).

You can’t go wrong with any of the suggested lights in this post. Don’t hesitate to pick one up and get your pond back. Any thoughts or questions would be greatly appreciated. Please leave a comment below. Until next time, enjoy your backyard water garden!


About the Author Jeff

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