Achilles tangs come from hawaii and as far as I know there are no cyanide caught fish coming from hawaii so cyanide is out.
One of the big problems with achilles (after you get past the poor shipping record ) is getting them to adjust to captive life, eating prepared foods is one of those things the tang is going to have to adjust to...many won't adjust and they die
. Having 3 other tangs in the tank won't help either