Update
Before checking Raspi-Config, I physically unplugged my sensors from the board. Rebooted and plugged back in. Reef-Pi now sees the sensors = win.
BUT because I deleted my "Tank Temp" from the Temperature tab, I needed to set that up again with Temp control. I can't save the settings because the UI tells me "The temperature settings cannot be saved due to validation errors. Please correct the errors and try again. " in the pink box. I also get the "hysteresis_less_than" notification in red under the Hysteresis box. I have tried various numbers for hysteresis.
Reef-Pi is measuring temp and plotting the graph now, but I can't add control.
Solution?
Discovered something.... When setting up Temperature control, you need to define Chiller Threshold as well as Chiller in addition to defining your heater, even if you have no chiller attached. If not, you get the "Hysteresis error" described above.




