Originally Posted by Lion Mom
Why? ALL of my submersible, adjustable heaters are in my tanks that way and have been for YEARS. Much easier to hide that way.
I don't think there're any functional problems with a horizontal heater, but there is an advantage to placing it vertically. It'll be more convenient to adjust the temperature setting because you won't have to immerse your whole hand into the tank water.
Placing the heater in the corner of the tank is also not too good if there's no water flow, as the areas far away from the heater will be significantly cooler.