The 12V SMPS power supply is not truly fully isolated. There is some capacitance to the Neutral and Hot lines of your AC power.
Precisely. The tank is likely already grounded. By connecting the 5V supply to the tank's contents, you are probably setting up a current loop through the ground. Things work on the battery because the battery is isolated from earth ground. But once you use the power supply, now you have a direct or capacitive connection to ground, and the 5V connection causes problems.Simple test: simply disconnect any 5V supply leading to any tank probes, and see how it runs. Of course, the tank level measurement won't work, but does it still exhibit the reset/freeze issues?
Sankalp, See my very first comment in reply#4. Did you try this?