That chemical won't harm your fish, although it affects only a small percentage of the fish in the tank where it's used. It's best to use it when your tank is full to increase the chances of seeing something happen. It works immediately, so any effect will be observable right away. Depending upon what fish are already in your tank, it's possible to get Magic Fish that way.

There have been several very good discussions about it already. If you search for "unknown chemical" in the forums (make sure that you use the quote marks and change the date range to 1 year), you'll find more than 50 posts that discuss what it does and how people have used it to their advantage.