A combined ultraviolet light and ozone fluid sterilization system for sterilizing fluid that includes a removable and replaceable internal reflective sleeve is provided. The sterilization system includes a lower housing, an upper housing, a winged nut, a UV light manifold, a plurality of UV light assemblies, a plurality of UV light securing assembly, and a reflective sleeve. The UV light assemblies include a UV light and an ozone siphon pipe positioned within a quartz casing, which is sealed with an endcap. The ozone siphon pipe of each UV light assembly can be operatively connected with a venturi for introducing ozone into the fluid. The sleeve includes perforated ends which create a more uniform flow within the sleeve, reduce air pockets, normalize the residence time of the fluid molecules, normalize the velocity of the fluid, and increase overall uniformity of treatment.