Leave it to Takara Tomy to come up with yet another robotic toy that does not need tending. The Robo Fish depends on two watch batteries that propel it up and down. That movement makes it appear that it is randomly rooting around for food. So far only available in Japan, the fishbots come in 4 different models and carry a MSRP of ¥2,980 (~$38.00)
On the other hand, if you don't want to wait for it to be imported, Fincredibles put out a Fish Bowl with a pseudo clown fish that will annoy swim endlessly with an additional benefit of changing LED lighting. The electronic bowl needs 3AA batteries (included.)