Could you describe the sequence of events and the resulting state of the machine in a bit more detail? For example, when the screensaver activates, the screen(s) fade to black (do you have one monitor or multiple monitors?), then the aquarium appears. Do you see the fade to black? Does the aquarium appear? Does it start animating? Is sound on or off? If it's on, do you hear it? When it freezes, what (if anything) is on the screen(s)?
Do you encounter the same problem when you run the application? Have you tried running the application in full-screen mode? Any problems when doing so?
You said the screensaver works fine when you use the Test button in System Preferences. Do you have a hot corner defined to activate the screensaver? If so, what happens when you activate it using the hot corner?
When is the screensaver set to activate? What are your sleep settings in the Energy Saver prefs panel?
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