My understanding is that they aren't literally migratory, though that could be true as well. But their environment is so changeable that they might as well be; they live in intermittent streams in a monsoon zone, so temperature, hardness, cleanliness, water flow, clarity, and depth all vary widely with the seasons. That really fits with the "easy to keep but hard to breed" characteristic: They can adapt to just about anything within reason, but recreating those seasonal variations in a fish tank would be a great challenge, and wouldn't facilitate the kind of eye-candy tanks most of us enjoy. Sure would be fun to try, though.