Since one of the most important aspects of a Fish Tank Virtual Reality (FTVR) system is how well it provides the illusion of depth to users, we present a study that evaluates users' depth perception in FTVR systems using three tasks. The tasks are based on psychological research on human vision and depth judgments common in VR applications. We find that participants do not perform well under motion parallax cues only, when compared with stereo only or a combination of both kinds of cues. Measurements of participants' head movement during each task prove valuable in explaining the experimental findings. We conclude that FTVR users rely on stereopsis for depth perception in FTVR environments more than they do on motion parallax, especially for tasks requiring depth acuity.