05-01-2012, 03:18 PM
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It doesn't matter as long as it's under water up to the designated point for thermostatic control. It's also best that it is centrally located or where water circulation will ensure a consistent temperature throughout the tank.
Also, it can be handy to have it low enough so it's below the water line of your weekly water change. Mine are high, but I turn them off before doing changes - always a good idea...but if you forget, having them lower better ensures they survive.
(I'm old school and in the olden days there was no such thing as submersible heaters. The whole idea of putting an electrical device under water still gives me willies!)