A purifying system for treating a fluid with UV light including a housing defining: a chamber having a treatment zone; an inlet; and an outlet. The light source supplies UV light to the treatment zone. The inlet, the tubular housing, or the outlet may extend inward into the chamber so that the fluid passes close to the light source and has a relatively increased duration in the treatment zone. The purifying system may also have a sensor coupled to the housing or light source for generating a first signal indicative of a first parameter of the treatment zone. A microcontroller can control the light source based on a signal from the sensor and data characterizing the light source. The microcontroller may also vary operation of the light source and collect data to create the data characterizing the light source.