As you should know, heaters are used to keep the aquarium water warm. You should set the heater temperature at the desirable temperature for the fish you have. Over heating could result in death of the fish and the plants your aquarium has. It is best to use two heaters for a big tank, having one at each end. Never place a heater into the gravel. It can result in uneven heating of the glass which could shatter or crack, allowing water into the heater. It's best to put the heater near a filter or bubble stone. This allows the heat to circulate throughout the tank.