Big Fish Games handles the registration codes for Fish Tycoon for Windows. Their help says that AOL and Hotmail may be blocking email from Big Fish Games. There is a link on their help site for special help for users of those email systems.

They have help for the registration process here, if you are still having problems or questions.

According to the help, it looks like Internet access is required to install the game on more than one computer. If you try to register Fish Tycoon without Internet access, and it fails, you may need to contact Big Fish Games suppot.