I bought 2 black skirt tetras along with some other fish for my 10 gallon tank. The next morning one of the tetras died, from stress I think. The other tetra was on its own for about 5-6 hours, then I added another black skirt tetra into the tank. Now the old one keeps trying to nibble at the new one's fins. He tore a piece off and every time the new one starts swimming around, he gets chased by the old one. And now I'm noticing he's started to nip at my other fish' fins.
What is the cause of this? How do I stop it? If I take the bully fish out and put him in a different tank for a week or two and put him back in the 10 gallon tank, will it make him less aggressive?
I'm scared the new one will die soon, which is sad.