That fish of yours has haunted my dreams!
I murdered a friend's poor fish once.
It had become trapped in something in the tank, and the other fish had eaten most of its fins. Awful. Once released, it couldn't control its attitude in the water, swim remotely normally etc.
My friend had a big thing about not interfering in nature..(eg allowed her dog and cat to suffer inordinately before euthanasing them...I nearly took them off and had that done, thinking I might be able to pretend they had died...seriously! It was so horrible. I offered to pay and all.)
I noticed the other fish were slowly eating the injured fish alive. Its scales had become all ragged where they had eaten THEM, and I knew flesh was next.
I am afraid I killed the poor thing myself, and endured the storm of anger rather than letting this go on. We were sharing a house at the time.