What do you mean by flow rate and do I need to reduce the filter performance?
By flow rate we mean the actual water flow through the UV-C water clarifier. If you connect the UV-C device behind the filter, all of the water which is run by the filter flows through the water clarifier, and the germ-inhibiting effect takes significantly longer (with regard to the total water amount of the aquarium). With powerful filters either the separate installation by means of a small circulation pump or the operation in bypass mode is recommended. This means running the water from the filter outlet to a y-piece (separation of the water), with the water clarifier installed in one of the water cycles, and then you can reconnect both hoses over a second y-piece, before the water flows back into the aquarium.