Abstract: A wireless electronic monitor for a container such as an aquarium is described. The apparatus comprises a sense and transmit assembly (STA) 15 configured with a pH sensor 6 submerged in the water inside the aquarium, and a receive and display assembly (RDA) 33 that displays the output of the sensor 6. A line-of-sight orientation is maintained between openings in each assembly using magnets to generate a clamping force on a transparent tank wall 16. A water test button 22 is pressed, and a single pulse of light travels from the RDA 33 to the STA 15. The single pulse of light turns the STA 15 on by closing a timed switch to the battery power. The pH sensor 6 output is converted to a train of frequency-modulated pulses of light that are transmitted back to the RDA 33. The frequency of the train of light pulses is determined by a CPU in the RDA 33, and assigned a pH value from calibration tables stored in electronic memory.