Dude so sorry to hear it, sounds like you're pretty upset. I would be too. How long has he been like this, any idea? He's a big fish, can go for quite a while without eating. Pour in some melafix and stress coat. He may surprise you. Don't give up on him just yet. As for who did it, you would know best. I don't have any experience with flowerhorns but I understand they can be rather nasty. if yours is usually hiding, he may have staked a serious claim on his favorite spot. The TX maybe was pushing the limits? Don't know. Don't give up. Give him a few days rest, maybe even a hospital tank if you are able. In a few days try some blood worms or something else he will have a difficult time resisting. Chin up.