In this paper we introduce new user interface concepts for fish tank virtual reality (VR) systems based on autostereoscopic (AS) display technologies. Such AS displays allow to view stereoscopic content without requiring special glasses. Unfortunately, until now simultaneous monoscopic and stereoscopic display was not possible. Hence prior work on fish tank VR systems focussed either on 2D or 3D interactions. In this paper we introduce so called interscopic interaction concepts providing an improved working experience, which enable great potentials in terms of the interaction between 2D elements, which may be displayed either in monoscopic or stereoscopic, e.g., GUI items, and the 3D virtual environment usually displayed stereoscopically. We present a framework which is based on a software layer between the operating system and its graphical user interface supporting the display of both mono- as well as stereoscopic content in arbitrary regions of an autostereoscopic display. The proposed concepts open up new vistas for the interaction in environments where essential parts of the GUI are displayed monoscopically and other parts are rendered stereoscopically. We address some essential issues of such fish tank VR systems and introduce intuitive interaction concepts which we have realized.