Goldfish tail ripped up?

Yesterday i bought a Black Moor GoldFish. I have 1 Goldfish already and 1 buenos Aires Tetra. My tetra has gotten along with goldfish for ever since i got him. I had 2 goldfish before, 1 died recently, so i got a black moor.. and i checked on it today, and its tail is all ripped up like it was attacked!! I had a heart attack! I havent had it for even 24 hours and its already hurt! Was it the tetra? i have no idea...