The placement of the heater in the aquarium is of prime importance. Submersible heaters are best placed horizontally near the bottom. However, some can be installed vertically. For the best results, place heaters near a filter inlet or outlet. You can put them near a circulation pump or air pump as well. Heaters should be away from any decorations so that the water can flow freely. As the water flows in front of the heater, it will become hot and spread this heat to other parts. It is always better to have more than one heater in tanks and place them on opposite ends. This prevents any cold spot formation and results in more thorough heating. One should understand that heat travels in three ways, and an aquarium heater controller should benefit from them. * Conduction: heat spreads from hot to cold areas. Though water is a better conductor of heat it is not very efficient in a tank. * Convection: when water becomes less dense on heating, it rises, and cold water replaces it. * Circulation: this is how heat transfer takes place in an aquarium. The filter pump will circulate and mix the water to spread heat all around. You should place the heater near the water inflow or next to an air pump where water flows past the heater. These are the best locations for quick and even spreading of heated water in the entire tank.