The obvious ideas are often the best. This also applies to the aquarium heating element. It is simply hung in the water and heats it up. The principle is the same as it was decades ago. But in the meantime the EHEIM controller heater has become an ultra-modern thermal device. The temperature can be set precisely and is measured even more precisely and kept more constant by the electronics. The coating made of special laboratory glass increases the heating surface, serves as a heat shield and ensures even heat emission. 25W.