Atfs can and will attack other fish who look like them. Often this doesn't seem to be aggression, so much as displays of dominance and the like. Still though, when your display of dominance is to rake the entire length of the other guys body with your mouth-knives, the damage tends to be severe. I've found that ATF are remarkably resilient to the abuse they do to each other, but it can go poorly for the dithers. Still though, it's better to lose an $8 barb than an $80 ATF.