OK. I was thinking, essentially, you would have a tank that you could slip the iMac onto the side. Maybe use the existing iMac support leg but cut it shorter, giving it the effect of being there when viewed from front.
An interesting project anyway! I googled around some, many examples of G3 iMacs as aquariums, only the for a G5
http://www.youtube.com/watch?v=u2GQl6SHcL8