Kessel, A. L., Brent, L. 1996. Goldfish as enrichment for singly housed chimpanzees. Animal Technology 47, 1-8.

The aquarium [with a single goldfish] was attached to the outside of the chimpanzee's cage ... so that the single-caged subject was able to touch the aquarium but not the fish itself. At the conclusion of this study the authors decided that it might be more beneficial to provide the goldfish on a more intermittent schedule in an attempt to alleviate the 'novelty effect'. This is currently being accomplished by having our enrichment technician schedule 'fish' as an enrichment option one day each month. ... The authors also felt that the provision of a social group of fish might also be more interesting to the chimpanzees as well as being more enrichment to the fish.