Will the mirror help for my FH?

1 have 1 juvenile FH 2-3 inches in length. My FH gets excited or somehow angry when he sees his own reflection. Some say this is good for his kok and his color. But some says that this causes stress for the fish. i would like to know the rationale of how does the kok improve when FH's are excited and do fh's should avoid seeing other fish or their reflection?