Get rid of it now, while there is a possibility that you can catch it, and a slim possibility that a fish store exists that would be stupid enough to trade you something for it.
It's a neoglyphidodon melas -- black damselfish (commonly referred to as bluefin damsel also)
When they are juveniles they are stunning, one of the most attractive damsels IMO.
Then they grow and get uglier and uglier (kind of like when you have watercolor paints and mix them all together to make a mucky brown/black/gross color), and grow to be quite large, easily 3+ inches.
Don't trust any fish store that will sell you these fish. They also eat soft coral.