It has something to do with their fins and you most likely won't be able to see it on juveniles. I purchased 8 of them for my 250 gallon reef, about a year ago. before no time at all, I am down to 1. There wasn't another mean fish in the tank and non that could eat them whole. They simply picked each other off so fast, I couldn't do anything about it. As much as you see them school when they are young, they tend not to do this in the home aquarium. I've tried 3 times, all with 6 or more at a time, and all with 90 gallon tanks and larger. each time, was down to 1.
However, good luck with your purchase - if you decide to go that route!!