Sensors located at the top and bottom of the tank read water pressure, temperature, and level, and safety switches shut the system down if the water level gets too high or too low, respectively. (ftiinc.org)
I logged the top and bottom of tank temperature, ambient temperature, collector temperature, and solar radiation levels. (builditsolar.com)
If alone, especially after repeated periods of isolation, the fish would «freeze»: just sit at the bottom of the tank (see video). (scientificamerican.com)