Heating a open top tank

That's a little vague... what power is the water heater? What power is the heating element in your tank? Is this a fixed installation (requires a hard connection -- no plugs -- according to code) or does the heater in the tank float around, like a stock tank heater?

dipper said:

Hello all ( new here) question please I have a open top tank I need to keep at 90 degrees I’m wanting to put in a heating element with a thermostat I have a hot water heater in my shop now and I’m wanting to know can I just unplug it and use the power cord to run the element I’m putting into my tank

Why do you want to do that? There may be a better way of achieving your purpose.

90 deg F (32 degC ) is a nearly ideal temperature for cultivating legionella ( or any other random bacteria) in your tank. Also, immersion heater thermostats can fail by arc-welding themselves closed (permanently on) and the water may boil. If it's a plastic tank, it may soften and collapse, releasing a boiling waterfall. That has happened.