You could take it back to the shop where you got it from or get more of them. They need to be in groups of at least 6 (preferably 10) or more. Depending on the tank size (length x width x height) and water chemistry (pH, GH, KH) will determine if you can keep that species in the aquarium. They need a minimum size tank of 30 inches long and 3 foot or bigger is better.
The reason it seems angry is because it's a schooling fish and is stressed out by being on its own.