Heat and cool storage tanks

[ssctsa1]

I need to be able to heat/cool the liquid in my storage tanks to temps as low as 30 deg F but as high as 90 depending on the requirement. My tanks are already built so I cannot use an internal heat exchanger and I am not able to circulate my fluid either. Does anyone know of, or used, an external chiller/warmer on a tank to adjust temps within a reasonable amount of time (4-6 hrs)?

 

[civilperson]

Enclose in a insulated box and raise the exterior temperature.