I guess if you are really quick with your fingers, you can try to use the universal Mac screen capture keyboard shortcut (dunno if this works on Mac OS X):
Command (
)-Shift-3
It will dump a screen snapshot named "Picture 1" at the root of your main hard drive.
Assuming this keyboard shortcut works on Mac OS X, if you press it fast enough maybe you'll get a shot before the Aquarium exits.
I think if your turn off "Exit on Mouse Move" this also prevents stray keypresses from closing the Aquarium (only Esc or Q works). You might also have better success running the Aquarium in a large window instead of fullscreen when trying to take a snapshot.