El-Fish

A fish and aquarium simulator, El-Fish lets your 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 El-Fish 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.

El-Fish