It's a Giant Danio. It will grow to about 4", but aren't normally overly aggressive. They are a schooling fish, however, so being alone may increase it's aggressiveness. I would advise you to either rehome it, take it back to the store, or acquire 4-5 more to form a school. They actually go very well with Australian Rainbows if you want a small school of each and your tank is large enough. I used to keep both in a small indoor pond/fountain, and they did great together.