I have the same problem. Back on 11-Oct-2008 I purchased a version 2.6 upgrade, and that keycode has worked with EVERY version 3 beta through 9m. Now it prompts me to enter a keycode (in the "Register" dialog) and, when I do, it says:
Please enter "TESTFISH" or a valid keycode or place a Marine Aquarium 3 CD into the drive to continue.
I don't have a "Marine Aquarium 3 CD". I do have an Aquarium 1.0 CD, but it doesn't work.
It is a 2.6 upgrad code I have sent e-mail to PPi.