And I quote:
Fish School employs two techniques long used to train animals - positive reinforcement and shaping. The basic idea is to reinforce successive, increasingly accurate approximations of a desired behavior. In training an animal to press a lever, for example, simply turning toward the lever will be rewarded at first. Then, only turning and stepping toward it will be rewarded. With a little time and repetition, the animal will learn to perform the complete behavior to receive the reward.
It is a commercial venture (or it appears to have become one somewhere along the line), so I'll let you draw your own conclusions. However there are some pretty cool videos of fish performing tricks at the above site.
Here is a link to part of a radio segment about the people who created the Fish School site. It explains how they came to train fish, among other intricacies of the operation.
When I get my betta, he shall be an agility champion.