The thermoregulating device, especially designed for aquariums, includes within a waterproof envelope, a heating element and a working unit regulating the temperature, moving axially in order to vary the rest position of a bimetal strip which, depending on its deformation, opens and closes the supply circuit of the heating element. The regulation unit (3) is provided with a threaded pin (13), the end resting against the bimetal strip (7) and with a cylindrical body (11), provided on the side surface with a spiral-shaped line (12) covered by a screen (18) provided with a longitudinal slot (19) through which a short portion of said spiral-shaped line (12) is visible.