Abstract: The invention is concerned with a pet animal structure for insertion into a transparent tank. The structure is in a single piece with upper and lower body parts. The lower body part has floors for movement of the animal and the upper body part has a shoulder which overhangs the lower body part. The upper body part when the structure is inserted an appropriate size tank, closes and extends into the top opening to the tank with the shoulder below the top opening so as to protect top regions of walls of the tank. Preferably, the upper body part has a downward facing support surface for supporting the structure upon a top edge of the tank so that the bottom of the structure is spaced from the bottom of the tank. Also advantageously, the floors are positioned to prevent the animal from gnawing at the shoulder while providing freedom for the animal to move between the structure and the tank when standing upon the bottom of the tank.