One of mine used to go really pale and not eat when I removed his neighbor, and it had to be this ONE, SPECIFIC fish. It couldn't even be the brother of this specific fish, who looks a lot like him...it had to be that fish.
And there's another one who has to have his favorite male Crowntail next to him. They flare at each other all day long, day in, day out. It's their favorite activity. I used to have to block their view of each other just to get them to eat. But I block their view at all for more than a few seconds, and the Veiltail goes down to the bottom of his tank and SULKS. He won't eat...he just lays there on his side and absolutely sulks.
I really think they're much happier when they aren't entirely on their own. All mine are kept at least in pairs for their own sanity. They're just more active with that kind of stimulation. I do separate sick males, though, and put them next to more sedate males, who provide company without making the sick male feel like he has to fight with the other male.