The first rule of thumb with an aquarium heater is the need for it to be completely submerged. Some heaters will mount inside the tank vertically, others horizontally, or both. Place the aquarium heater in an uncluttered corner of the tank away from greenery or decorations so that nothing will interfere with the heating core or prevent heated water from properly circulating the tank. Check the instruction manual for the best position for your aquarium heater.\r