An aerator assembly for an animal watering device having a rotor mount removably inserted in a discharge opening of a cover of the device and a rotor carried by the mount that is rotated by discharged water. The mount includes a conduit having one end inserted in the cover and an outlet at its opposite end conveying a discharged stream of water against rotor baffles causing rotation. The mount includes an arm that offsets an axis of rotation of the rotor relative to the stream so water impinges against a baffle. A pin can be included that engages the arm that is received through a rotor hub rotatively attaching the rotor to the arm. Such an aerator not only aerates water discharged during fountain operation, it also reduces noise by reducing splashing. The present invention is also directed to a pet fountain equipped with such an aerator assembly.