I agree with TestDrive, you should reassemble the unit and test whether ethylene glycol is compatible with it, at the same time testing what temperature it will ultimately reach. If it reaches 200 F in a reasonable time, you could use it to heat a smaller reservoir and have that bleed into a larger reservoir for a longer lasting defrosting capability.
Like this: (sorry about the crudeness of the picture)
Tank heater setup (Small).bmp