Hydor eth is the first and only heater to function outside the water thus guaranteeing a regular well distributed temperature controlled by an electronic circuitr eth means also complete safety and no more power cords or accessories to spoil the look of your aquariumr how does eth workr eth is connected to the outlet hose of any kind of external canister filter the water already filtered is pumped towards the tank through the heater activating a dynamic transfer of heat thus ensuring a c