In general, the best and most common placement for a UV clarifier in a pond is just after the filter. Many biological filters come with built-in UV light, and it is important to place the UV light behind the filter when installing a separate unit. This will prevent debris from entering the UV chamber and damaging the inner glass. A UV water purifier should always be installed after other filters in the chain.
This is because they don't actually remove any unwanted material, but rather act as sterilizers by killing algae. However, algae remain in the water because UV clarifiers don't remove them. It is also important to install a UV water purifier after all types of water treatment systems in general. If green water algae still appear despite having UV lights in your pond, it may be because they are blocking the rays from reaching all the microbes that pass through the filter.
UV pond lights or UV “filters” are ultraviolet light systems that have been used for years in ponds and aquariums with the aim of controlling green waters caused by algae. To install the UV clarifier in line with your pond tubing, you'll have to cut the tubing and place each end of the unit on each side of the tubing you cut so that, after the job is done, the UV clarifier essentially serves as a continuous part of the tubing. While UV lights are good for removing algae from water through sterilization, this same process eliminates both good and bad bacteria. In general, regular doses of bacteria in your pond will be much more effective for ponds than UV lights, as they provide a biologically balanced pond.
That said, if you had to choose between a filter before or after light, it is recommended to have a filter before UV light. Keep in mind that the bulbs may still be on, but this doesn't mean they're still emitting UV rays, so you need to consider the quality of your lighting system. In an environment where UV light is the last stage of the in-line filtering process, dead algae clump together and are then released into the pond. It is essential to understand how to properly install a UV clarifier in order to get maximum efficiency out of it.
Placing it before or after filters can make a big difference in its effectiveness and should be taken into consideration when setting up your pond system.