Copy & paste time again kiddies. My humblest apologies of course:
Heaters should be mounted at 45° angle: You see the thermostat is at the top of the heater. As the heater heats up the water it will rise and if mounted vertically the just heated water will go straight past the thermostat giving the thermostat a false indication (or more importantly an unstable indication) of the temperature of the surrounding water.
It is the heat of the water of the entire tank you are trying to target & stabilize, not just the immediate locale of the heater. Also for this very reason the heater should not be placed in a stagnant region of the tank, better still in a fast flowing region. Again the heater should be placed in the middle of the tank, not at the bottom, not at the top so that the temperature sensed by the thermostat is a true meridian.
Similar situation to having a thermostat in a house mounted above a radiator. Yes the heat you set is correct for immediate area, but in the rest of the house it is all over the place. Conversely if the thermostat is one end of the room and the radiator the other, and then the temperature you set on the thermostat will be a truer indication of the temperature of the room as a whole.
You don't want it horizontal / flat either - hopefully it's placed it in fast flowing water, so make the best of the volume of water flowing past it.
All pretty anal of course. Some manufactures of heaters state that the heater should actually be mounted vertically....
Andy