sump if you have one. if not, any inconspicuous place on the back wall of the tank where there is significant current. a lot fo people like to put them in corners. I don't unless there is current in the corner (i.e. a powerhead is blowing into that space). i did see a good heater made particularly short and fat so it could sit in the well of a HOB filter like a penguin.... sounded interesting, but I like my ebo-jager too much.