Aquarium heater placement should follow the manual for the exact model. In many setups, the goal is to place the heater where water circulation can distribute heat through the tank.
Use circulation, not a stagnant corner
Manufacturer guidance commonly places heaters near a filter inlet, outflow, or other source of circulation. This helps avoid measuring temperature only near the heater while another part of the aquarium stays cooler.
Use a separate thermometer
Place a reliable thermometer at the opposite end of the aquarium. That gives you a more useful check of the water temperature experienced throughout the tank.
Follow model-specific instructions
Submersion level, orientation, water line, and acclimation time differ by heater. Read the manual before mounting, unplug the heater before handling it, and do not assume every model is fully submersible. See Can Fish Tank Heaters Be Fully Submerged Safely? and What Size Fish Tank Heater Do I Need?.
