Well the Aquarium does not come with a good help file and there is no dialog box suggesting the existance of a Settings or Keyboard Shortcut window.
I think on first launch after registering, it should present a dialog
Thank you for registering SereneScreen Marine Aquarium. You now have access to a Settings panel to select other fish and control lighting conditions, and you can also find out about keyboard shortcuts to cause different things to happen in the Aquarium.
You can always bring up the Settings panel later by pressing the SPACE bar.
{Settings} {Keyboard Shortcuts} {Return to the Aquarium}