Create some actions, like move towards, move away from, (try to) eat, mating call, mating and maybe others. Let users create what creatures can learn, and if it's a new creation name them. And then place them in aquariums.

Then, the creatures should remember last X number of actions and then rate it depending on how much energy it has. Then remember the Y most successfull action sequences.

On repeated sequence, increase or decrease score depending on the new score.