Keeping fish in an aquarium is a highly popular hobby despite the fact that fish are not contact animals like dogs and cats. Practical and easy to keep, aquaria provide an opportunity to observe a microcosm, behaviors involving competition, mating, reproduction, territorial defense, and care of the young. Because of this the office aquarium can be an effective therapeutic device for the psychotherapist, since it invites relaxation, reverie, and unconscious associations with projective implications in important areas of concern. Unlike standardized projective techniques, the aquarium with its constantly changing panorama elicits highly individualized rather than normative data. The patient is apt not to experience the anxiety of a testing situation in talking about what he sees but to give free rein to his fantasies. This makes it useful in work with children and as a basis for directed exploration of fantasy within groups, as well as for stimulating unconscious responses in individual adults.