Elfish

A fish and aquarium simulator, Elfish lets you catch, breed, evolve, and even mutate tropical fish, then render them and let them loose in an aquarium that you design.

Except for a few pre-canned graphics and animations, all fish, plants, and even backdrops and floor gravel in Elfish are generated by the user using genetic algorithms that simulate real life in both appearance and movement. Even the music you want to play in the background can be generated with one of eight musical styles.