A fluid treatment system includes plural housings (10, 78). Each housing includes quartz tubes (16, 80) fixedly supported at an upper plate (12, 106) while being movably supported at a lower plate (14, 104). Each tube has a UV light emitter that can eradicate microorganisms in a liquid. The tube support arrangement includes a plurality of tube retainer assemblies. Each retainer assembly includes a tube holding cup (30, 90) surrounded by a coil spring (24, 94). Each cup includes an upper lip (34), a lower lip (36), and a foot (42). The lips retain the cup in a lower plate aperture (20,116) while the foot engages the tube's domed end. A cup can hold a tube in the lower plate while constraining lateral movement of the tube, providing spring-biased axial support to the tube, and allowing axially-extending thermal expansion of the tube.