Top quality immersion heaters are made by Phillips & Temro or Kim Hotstart. Avoid heaters made by Five Star.
Coolant immersion heaters function by boiling the coolant adjacent to the heater element. Hot coolant rises (convection). Using a coolant solution with excessive antifreeze content hampers this process as the increased viscosity of a rich mixture, >50/50 results in slower movement of the coolant and can lead to more rapid degradation of the glycol which shortens the life of the coolant plus the electric element runs hotter leading to burn out.
If you use an external tank heater such as the Kim Hotstart or Phillips & Temro, be sure to size them a little larger in watt capacity as the external heaters will lose some heating value due to the external tank surface.
Retired Cummins (Fleetguard) Field Engineer