Hi there
The best way to ensure proper circulation is to; house the filter at one end, and either pipe the flow outlet down to the opposite end of the tank, or to have a tube/pipe running down the length of the tank (through custom holes in the dividers) with outlet holes to pump some water into each section. Either way you have a filter with an inlet at one end, and water being dispensed in the furthest section or all sections. If you have dividers with holes for circulation, all water pumped to the far end of the tank has to come back through all the sections to the pump because of displacement, leading to great circulation. If you are happy that you have good circulation this way, it doesnt matter where the heater goes. The important thing is to have a thermometer at the opposite end than the heater. If dipping in a thermometer, check at the heater end and the opposite end.
If your circulation is not that great (dead regions in the corners etc) do not put a heater in these sections, far away from the pump, as it may cook the still water around it. The safest bet is to put the heater right beside the filter, so the water being heated is being moved and mixed somewhere at least.
Hope the waffle helps
Lee
