In the graphic depiction in the instructions for use for JBL AquaCristal UV-C, the UV-C water clarifier is connected behind the filter in an aquarium, whereas it is connected before the filter unit in a pond. Why is that so?
A UVC water clarifier is generally always connected behind the pump in order to ensure a proper flow.
In the majority of pond filters (with the exception of pressure filters), the filter holder is higher than the pond water level and the water flows out of the filter and down into the pond. The pond pump is in the pond. This is why the UVC water clarifier needs to be installed between the pump and the filter unit, to prevent the water from flowing back into it and blocking it.