That's exactly the reason why it's given. Because they're carnivores and can't break it down, it passes right through their digestive tract and forces out whatever else is there, though fasting first makes sure that there isn't much else in there to make the whole process as comfortable as possible. I would obviously never give it long term because it provides no nutrition whatsoever. While I don't know much about fish, I know quite a bit about other animals and the same principles apply. :)
As I said, he's doing much better, so I'm pleased.