The Case Against UV Clarifiers

Many people believe that UV Filters will help solve algae or water clarity issues in their pond.  In some situations, the UV may help, but it can also hurt.


UV functions by killing organisms that pass through the filter.  What organisms are killed depends primarily on the intensity of the UV and the flow rate of the water through the filter.  The higher the UV wattage and the slower the water flow, the more organisms that UV will kill.  For instance, if you have a filter with a maximum flow rate of 30 GPM, it may kill planktonic algae and bacteria (beneficial and problematic), but it may not kill fish parasites and other organisms.  You may need to reduce the flow rate to something like 15 GPM to achieve that goal. 


A UV Filter that is designed to kill planktonic algae, but not higher organisms is called a UV Clarifier, whereas a UV Filter that kills most all organisms is called a UV Sterilizer.   Clarifiers are commonly used for ornamental ponds, but Sterilizers are more commonly used for Aquaculture applications, although there can be crossover applications, especially with high end Koi ponds to combat pathogenic organisms, like Ich.  Please note, this article is not about that application.


Planktonic algae are the main target for UV Clarifiers, because it is the type of algae that can turn the water green.  Planktonic algae are single cell organisms that float freely in the water and proliferate when excess nutrients become available.  When planktonic algae passes thru a UV Clarifier, it can be killed by exposure to UV light.  However, there are a couple problems with this plan. 


First, planktonic algae are not the only thing that causes water clarity issues.  If your water is anything other than green, the UV Clarifier will do nothing to help clear it.  And in some situations it will clear up the green color, but leave behind a brown cloudiness due to decaying organic material,


Second, the UV light can also kill off beneficial bacteria that proliferates in the pond.  Beneficial bacteria grows on all surfaces in the pond and helps process wastes and excess nutrients.  If you scrub the water clear of beneficial bacteria, you will have a harder time establishing a balance in your pond where biological filtration cleans the water.   This also commonly happens after an aggressive pond cleaning.


Third, after you kill off the planktonic algae and beneficial bacteria, there is likely to be a nutrient spike in the water because many of the organisms that had been processing the nutrients are now dead and decomposing. Nature hates a void, so when planktonic algae and beneficial bacteria are removed and nutrient levels rise, something else will proliferate, and that is likely to be filamentous algae, otherwise known as string algae.  This type of algae is unaffected by UV, because it is anchored to the sides of the pond and mostly does not pass through the filter. 


Finally, a pond with a properly designed and maintained filter will not have a planktonic algae problem, making a UV Filter unnecessary, or even harmful.  Therefore, it is our opinion that rather than treating the symptoms (planktonic algae) with a UV Filter, you should treat the cause, which is the nutrient buildup.  How do you deal with nutrient buildup? 


  1. Prevention: Do not overfeed your fish. Design or modify your pond so that ground water does not run-off into the pond, bringing in nutrients with it. Do not use mulch or exposed dirt around the edges of the pond where it may fall in.  Cut back dead vegetation and prevent leaves and other organic debris from accumulating in the pond
  2. Filtration: Talk to an expert about having a properly sized filter, water pump and water line for your application.
  3. Maintenance: Talk to an expert about how to clean the filter without killing it. Also, when you shut down the pond for winter, if possible, move your biological media into the pond to keep it alive and ready to go when you start up in spring.
  4. Treatments: If you do all of the above but still have excess nutrients, you can apply a Phosphorous Binder to tie up available nutrients, preventing the planktonic algae from obtaining its food source and turning the water green.


If you still want a UV Clarifier after reading all of that, give us a call and we will sell you one!