What’s wrong with my Electric blue acara?

He stopped eating and is constantly swimming against the glass and decorations, water temperature and quality is fine and I’ve been doing water changes and adding in aquarium salt. All of a sudden all the other fish are nudging him, this was NEVER the case before. He sadly passed on that night the video was taken but i want to know what to do next time something like this happens again. Can anyone help or give some advice?