ETH stands for External Thermal Heater. This in-line heater is designed for vertical use. Simply connect the heater with tubing to your canister filter or wet/dry sump pump by cutting the filters tubing and connecting each end to the ETH. To help reduce the amount of maintenance needed it is recommended that you install the ETH within the filters output assembly. Completely safe to use, there is no risk of damage, as this unit will shut off automatically if it runs dry. An internal heat system ensures that there are no temperature variations. Fits most canister filters. 300 watt model is recommended for tanks between 53 and 80 gallons and fits tubing 16 mm or 5/8″ in size.