I have my heater in one of my 4 systems right by the output "stack" of my reeflo 250 in the sump. Since the air input of the skimmer venturi is most likely cooling the water placing my heater right at the output of the stack fits with the heating and cooling general rule to heat your loss.
Not only that, the currents are quite high there so the heater gets alot of help to bump the temp up the 8 to 9 degrees of rise required to keep my system at 78-79 degrees.
So heat the loss.
Expose the heater to good flow.
And know the amount of the room temp to your system temp you need to raise.
Last part keep the heater off of anything that it can melt and even if it's a titanium like the Aquaheat keep it at the lowest point to prevent it from being run above the waterline during water changes if possible.