If you ever wanted to know what size of Tetra 145122 Tetratec HT Heater HT 25 has, here the data:
Overall length with control knob: 24.0 cm
Immersion depth min .: 18.5 cm
Diameter Heizröhrer: ca. 2,1 cm
Diameter adjustment head: 2.8cm
Incidentally, the HT 50 is approximately 2.0 cm longer
Whether the heater works on a long time, I can due to the short time, and the summer temperatures
too early to say.