A MacQuarium is an aquarium made to sit within the shell of an Apple Macintosh computer. The term was coined by computer writer Andy Ihnatko as a joke (a jibe at the outdatedness of the Macintosh 512K) but MacQuaria have since been built both by Ihnatko himself and by many of his fans, and some are even commercially available.

Ihnatko originally designed the MacQuarium around the Compact Macintosh-style shell; at the time early examples of this form factor (the Macintosh 128K, Macintosh 512K and Macintosh Plus) were completely outdated and thus turning a system into an aquarium was considered "the final upgrade" as well as most affordable way to get have a colour Compact Mac. However, some enthusiasts have since successfully adapted the translucent iMac case (to form an iMacquarium) as well as the Macintosh TV, the Apple Lisa and the Power Mac G4 Cube.

Andy Ihnatko's original construction was called a "MacQuarium," however, both "MacQuarium" and "Macquarium" are acceptable forms and may be used interchangeably.

Quoted: en.wikipedia.org/wiki/Macquarium
www.wired.com/gadgets/mac/multimedia/2002/11/56217?slide=2&slideView=2 (picture)
Tutorial to make your very own: www.cs.tut.fi/~ace/macquarium.html
Fansites for old macs as well as recent imac-fishtanks: www.theapplecollection.com/Collection/MacAquarium/ , macquarium.takebacktheworld.net/ ,
iMacQuarium: imacquarium.com/ , thunkdifferent.com/2007/09/25/the-next-imac-review/
G4 CubeQuarium: home.comcast.net/~jleblanc77/cube/