Why is my Blueberry Tetra swimming sideways and upside down?

PLEASE HELP! My poor fish which is a Blueberry Tetra (don't know if that's the real kind) all of a sudden has started to swim sideways and upside down. It's like he can't swim straight. I looked at his fins and they look a little torn up, but other than those problems he is still breathing, but I can't tell if he's eating or not. I have him in a separate tank so he can lay down and not get swished around! Please help me figure out what's wrong! I want to help him. Thank you!