Fish-School impresses me.  They claim they can teach you to train your fish to do tricks.

I don't doubt that some fish are bright enough.  Zachary used to have a female betta that would watch him until he fell asleep, before swimming away from his side of her aquarium.

(On the other hand, we once had some spotted green puffers who were so stupid they couldn't figure out how to eat, sometimes.)

It is a little weird, to me, that you'd want a fish to be trained...Seems like the best thing is to make sure they have an interesting enough environment, and to enjoy them behaving like, you know, fish.