Your goldfish may never truly be free, but if you had one of these infinity aquariums you can trick them into thinking they are thanks to a design that "loops the fish around in an infinite tunnel of water and glass." It's like a mini fortress of solitude for your little underwater friends—giving them a life precious semi-freedom before you unceremoniously flush them down the toilet. Not surprisingly, the "Swimming Around In Circles" aquarium is only a concept at this point.
Why not make it in the shape of a Mobius strip? [en.wikipedia.org] Now that would be interesting. A Klein bottle would simply be cruel though...
I can also imagine some interesting thing that you could do with big glass tubes. The cleaning thing would be an issue though.
Where I live we have some rich guy who has a transparent floor in his living room with goldfish. Every week or so the fish die and he just has his people put in new ones. Just another argument for eating the rich.