Actually you'll find that more and more websites are shying away from having a completely new download for new versions. I say have a single "update pack" download which includes all the freebie updates, but there's no need to replace the existing EXE download each time you add new fish. When the Aquarium first runs, after 5 minutes, it should bring up a window "Check SereneScreen.com to see if new fish are available (this is done without sending any personally identifying information)?"
Here's another thing...
Right now, if someone has a problem running the Aquarium, they contact SereneScreen tech support and then that support person is dependant on the user giving them the necessary information about what version of Windows, which video card, which video card driver, what running tasks, etc. to try to debug the problem.
In a future version of the Aquarium, there will be a "Request Tech Support" button. At this point, I believe the user will be prompted with a message...
In order to get the best possible tech support and to quickly identify solutions, we would like to do a quick analysis of your computer's hardware and software drivers. This information will be encrypted in transit and will not be used for any advertising or marketing purposes!
(Proceed) (Get help without checking hardware)
It would send which versions you have of Windows, DirectX, sound card driver, video card driver, and also check for the presence of nuisance/conflicting programs which may be running in the background and preventing the Aquarium from working such as NetMeeting and Lotus ScreenCam.