What is the name of the shark that can fit in a small aquarium?

You need to define "small aquarium" a bit more. You could be talking about a home aquarium, or you could be talking about an aquarium in a small town, in it's museum or science building. In the case of the former: None. In the case of the latter, there are a few, but they couldn't stay in there long, as the smallest of sharks outgrow all but the largest aquariums quickly. The smallest sharks I have ever seen in an aquarium were in the Monterey, California aquarium, one of the finest in the world. They were Leopard Sharks, about 24-30"long. The tank was the size of a two-story building that takes up half a city block (the Monterey Aquarium must be seen to be believed). I was told when they reach 30". they are released back into the sea.