Use the old Windows XP screensavers, including Aquarium, with Windows Vista and 7
With Windows Vista, these XP gems were summarily broken. Most of us didn't realize, as we were too busy dealing with Vista's despicable, retentive ineptitude, but now that we're onto Windows 7... well, wouldn't it be nice to relive some of those nice Windows XP experiences?
One clever guy on the WinMatrix forums (back in 2009, it must be said) has ported the Windows XP screensavers to work with Vista and 7. Unfortunately, Windows Dancer has been removed for copyright reasons, but the Original Screensavers pack (including Aquarium) is still available! Once you've unzipped it, you should have an EXE called MCE2005Screensavers.exe. I chose to unzip that file (using 7-Zip), as I don't like running EXEs, but many people have attested that the file is free of viruses.
You should then have a directory full of screensaver (.SCR) files. Right click your favorite and Install. It seemed a little buggy, with some screensavers requiring multiple Installs before they finally worked. But as you can see above, the Aquarium definitely works on Windows 7.