UV lights are used mainly for water clarity issues targeting single-celled algae (which makes your pond water appear green).
Oftentimes UV lights are referred to as being clarifiers (UVC) or sometimes sterilizers. The main difference is the acceptable flow rate of water (gph) exposed to the UV light (dwell time). The slower the flow rate passing thru & the higher the wattage, the more likely the unit is being used in sterilization purposes.
Typically even low wattage UVs can damage the outer cell wall of the single celled algae to allow it to stick (flocculate) to the damaged wall of another algae cell. This process allows it to be large enough to be caught in the filtration media.
UV light bulbs need to be switched out once per season as a bare minimum.
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