How to choose Pond UV Filters, Ultraviolet Clarifiers, and Sterilizers


There are 3 components discussed in this article:  Ultraviolet Clarifiers, Ultraviolet Sterilzers, and UV Fiters.

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One misconception of ultraviolet clarifiers is that they “kill” beneficial bacteria. Ultraviolet fixtures can kill beneficial bacteria when sized and used with that as the goal. Most ultraviolet clarifiers are too small, do not emit emough ultraviolet light, and have too high of a flow rate to kill beneficial bacteria. 

There are a lot of articles and information regarding UVC (ultra violet clarifiers) for ponds. There are also different names and different names, components, and uses for pond UV. First, lets name the different components: UV clarifiers, UV sterlizers, and a slang term given to a pump for filter with a UV is a UV pump for UV filter. Keep in mind, pond UVs do not mechanically or biologically filer pond water and therefore must be used with pond filters

All UV devices for ponds have a primary objective of keeping ponds clear. We have all experienced green water in our ponds and it can be extremely frustrating. Without ultraviolet clarifiers, we have to balance the pond with pond plants, pond filtration, algae control products for green water, and beneficial bacteria. Using a UV still requires us to use the above mentioned products, but, we use them much less and with better results.



Ultraviolet Clarifiers for Ponds (UVC)



Pond Ultra-Violet Clarifiers (UVC) prevent algae cells from blooming and multiplying thus eliminating green water from the pond. These devices expose water to ultra-violet light that performs the above functions. In years past, these devices were sold by explaining that ultraviolet clarifiers clump algae particles together to make them large enough to be picked up by the Pond Filter.

The ultraviolet clarifier accomplishes this by being placed inline with a pond pump as water must be pumped through the ultraviolet clarifier. Ultraviolet clarifiers have “maximum” flow rates that must be adhered to because if water is pumped too fast through the ultraviolet clarifier, the algae suspended in the water that is blooming will not be exposed to the ultraviolet light long enough to kill the algae.

In most cases, installing ultraviolet clarifers inline with the pond pond or waterfall pump will slow the flow of the pump down. In other cases, the main pond pump or waterfall pump may be too large for the maximum flow rate of the UV needed for the pond size. In this instance, it is best to purchase a pond pump dedicated to the ultraviolet clarifier. For example: if a pond has a 4,000 gallon per hour pump, and the pond is 1,000 gallons, a 9 watt UVC will do to keep the pond clean. The max flow rate on the 9 watt UVC may only be 900 gallons per hour. Therefore, it makes sense to purchase  separate pump for the 9 watt UVC rather than purchase a UV that can handle 4,000 gallons per hour.



Ultraviolet Sterilizers for Ponds (UVC)



Ultraviolet Sterlizers are primarily installed on koi ponds where harmful bacteria and protozoa are a major concern along with algae that turns ponds green.



The Difference between UV Sterlizers and Clarifiers



UV sterilizers are typically longer, a higher wattage, and more of a commercial or contractor grade type of product. In many cases (not all), they are designed for a professional to install. Being longer in physical size and of a higher wattage, water stays in the UV chamber longer thus being a cure for bacteria, protozoa, and single cell algae particles.

UV Clarifiers typically have adapters for flexible hose ranging in size from 3/4″ to 1.5″, are more compact, do not emit large amounts of UV, and are designed to fit a multitude of DIY applications.



UV Filters and UV Pumps




UV Filters and UV Pumps are just like they sound: UV filters are filters with an integrated ultraviolet clarifier and UV pumps are pond pumps with UV. UV filters are filters with a UV in the top. The water travels though the pond filter first exposing pond water to mechanical and biological filtration before exposing the water to an ultraviolet clarifier. Water travels through a UV filter in this manner o that beneficial bacteria are not damaged when added to ponds with a UV filter.

UV pumps are a great idea, thousands are sold each year, but, there is a drawback to UV pumps. In many cases, one of the two components fails rendering the UV pump useless. Unlike a UV filter, parts for UV pumps are not readily available. For example, if the pump motor goes out, there is no replacement pump for a UV pump. The same is true for the UV component of a UV pump. Where as with a UV filter, or a pond filter with a UVC, most of the components can be replaced. The same is true for an “all-in-one” pond filtration system. This is especially true if the pond filter is purchased though an independent. Beware of online marketplaces such as and, too. It’s rumored that any warranties after 30 days for products purchased though these online marketplaces must be taken up with the manufacturer. This can be a daunting task. An independent pond supply store will handle the warranty service for you.



Components and Care of Pond UVs



Ultraviolet Clarifiers and Ultraviolet Sterilizers for ponds are typically comprised of the same components: the body, transformer, quartz sleeve, and UV bulb. The quartz sleeve is what sits in the body and keeps water off of the bulb. These two pieces, the quartz sleeve and UV bulb, are easily damaged by freezing conditions and therefore should be brought indoors during winter months.

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