I had a battery powered aquarium for a while; I wish I still had it, I've never seen one as nice since.
In mine, you had to actually make up salt water to put in it (so the fish didn't all sink), which was fun (it only held about a pint of water) and there were a few different species. I remember I had two clownfish (a black and white and a red and white) a powder blue surgeon, and a Moorish idol.
The thing I liked most was, I suppose because of their slightly different shapes, they actually behaved differently! The clowns sort of went round and round a piece of coral, while the surgeon stayed mostly up at the other end and the Moorish idol used to swim from side to side!