It is not true that everything that is plugged in uses power even when it is switched off. Any appliance that has a mechanical on off switch that opens contacts in the supply to the system uses no power at all when that switch is open (off). This is how the cheaper heaters work, they have a bimetallic strip which mechanically opens the switch when the water excedes the temperature then no power is used at all, NONE, not a sausage. This is the same as room lighting with a traditional rocker switch.
In systems which have an electronic switch the answer is different as the switching system itself generally consumes some power even when the appliance is off, some more expensive aquarium heaters work this way, it is a fair bet that anything labelled electronic or with tronic in the name falls into this category. I would not worry about the power consumption though as it should be very low when off and the improvement in reliablity and life of the unit is likely to be far more important than any power consumed while the heater is off.
