Hello! I have a 55 Gallons tank with 5 Black Skirt Tetras, 2 White Skirt Tetras, 2 Long Finned Red Minor Tetras. I got them on Friday (01-04-13), and the Black Skirt Tetras have been aggressive towards each other and the other fish in the tank. 4 of the 5 Black Skirts have missing fin pieces...