A system for disinfecting fluid includes a UVC LED. The UVC LED includes an LED chip configured to emit UVC radiation and a package coupled with the LED chip. The LED chip has a top surface that defines a chip top surface area. The top surface is formed from a semiconductor material having an index of refraction. The fluid reactor has at least one wall that defines a chamber configured to contain the fluid. The at least one wall has an aperture configured to receive UVC radiation into the chamber. The aperture extends through the at least one wall. The aperture has an aperture area that is (1) smaller than a top surface area of the package and (2) equal to or larger than the chip top surface area.