The site above features a very nice collection of mini-aquariums, along with code for you to include them in your own website or blog, may you want to. I tested a miniaqua in nan´s sidebar for a few days, and after “installing” it my browser updated (automatically) the Shockwave player. Then the aquarium ran perfectly, though a friend told me today that he had been unable to open the site, so be careful if you decide to use it. By the way, there´s another version of the aquarium available from the same site, though it´s not as mini 😛
Now, for some real-ly small fishtanks, you may like to take a look at this selection.
Related Article: The megatank!