Originally Posted by Vircomore
Mounting is really specific to the model. If they say not to, then dont.
However you should take a close look at the picture - my Fluval E had several pictures that showed how to "NOT" mount the heater. One was Vertical and one was horizontal, until I looked closer and saw that what they were saying was "don't hang the heater by just the cord" and "dont just lay it on the bottom".
Ah right, I'll have to have another look at that!
I do remember reading something that the thermostat was on the opposite end to the heating element, therefore if you mounted it vertically the heater water would rise straight up and the switch the heater off, so you'd never get the tank up to temperature.
I'm not sure what the reason for not mounting it horizontally is, apart from that you usually want the heating element to be as low down as you can and the wire to be as high as you can for each reach of the plug!