Very interesting. I know that several goldfish have been taught to navigate mazes, respond to certain colors, and ring bells, but I haven't heard much about training bettas. It doesn't surprise me though; I think people are under the impression that just because an animal is aquatic, it is by default stupid. Yet if you look at the behaviors of fish, they closely mimic terrestrial animals: there are group animals and solitary ones, predators and herbivores, parental care and a lackthereof, courtship displays, territoriality, and so fourth. I would suspect that many fish are absolutely on par with a simple mammalian species.
My bettas don't know at tricks, they just know to follow my fingertip to wherever I want to feed them.