In a support device for aquarium accessories including a first fixing element appliable on the inner surface of a wall of an aquarium and a sustaining element for an accessory supportable by the first fixing element, the first fixing element has a concave cap and the sustaining element has a convex cap housable at least partially inside the concave cap. Furthermore, the support device includes a cap-shaped first magnetic or ferromagnetic element, the first fixing element includes a second magnetic or ferromagnetic element, and the convex cap of the sustaining element is interposed between the concave cap of the first fixing element and the first magnetic or ferromagnetic element so that, due to magnetic attraction between the first magnetic or ferromagnetic element and the second magnetic or ferromagnetic element, the sustaining element is supported by the first fixing element and, at the same time, it is rotationally movable.