Eheim Jager HeaterAs you know, fish from tropical and sub tropical waters require a constant water temperature. Before the engineer Eugen Jger invented the aquarium adjustable heater some decades ago, there wasnt any really satisfactory solution for controlling the water temperature suitably for specific species. People resorted to complicated and sometimes odd methods. Some put the aquarium in the sun or next to a radiator or an oven. The EHEIM aquarium heater is a