I added 6 Black Phantom Tetras to my tank 2 days ago. Their only tankmates are 8 zebra danios. The tank is 125l and ammonia and nitrite are both sitting at 0ppm. Today I noticed ones of the Male fins appeared discoloured and he looks like there is a tear in one of his fins. As I am new to the hobby I am concerned that he may have some disease, or am I overreacting and he may have simply came off worse against one of the other males I have. Thank you for any advice