This thread contains the links to download the latest Installer and Standalone versions of MA3:
Marine Aquarium 3 Beta NEWS & DOWNLOAD
You can
SUBSCRIBE to that thread to receive an e-mail any time there is an updated version. There is currently no way to get an e-mail ONLY when the Installer version is updated.
It wouldn't do to subscribe to THIS thread, as it is a discussion thread and gets lots of hits during the month.