As ingenious as it is simple water gets sucked in filtered and heated at the same time and then pumped back out at the correct temperature into the aquarium the eheim 2178 professionel 3e external filter 600 thermo is a very high quality electronic external filter with integrated heating system ihs for freshwater aquariums from 300 600 litres capacity all the functions like water flow or flow shifting can be set with just three buttons on the eheim 2178 professionel 3e external filter 600