An accessory device (1) for an aquarium is described, in particular for a thermal heater (2), which may be vertically positioned. It comprises an accessory device (2) for operation of said aquarium and supporting means (4, 5) for supporting the accessory device (2) on a wall of the aquarium tank. The supporting means (4, 5) comprise a supporting body (4) which may be attached to the aquarium wall and latching means (5, 10), which are integral with the accessory device (2) and may be removably associated with said supporting body (4). The supporting body (4) supports the latching means (5, 10) and the accessory device (2) and allows selection of vertical position of accessory device (2) with respect to supporting body (4).