When I had my GATF I found that one of the only fish that he wouldn't mess with was green tiger barbs. My g/f was changing her tank from community fish to africans. So we just tossed her old stock in the GATF tank for food and he took down guppies, mollies, barbs( lemon, cherry) and some others but it wasn't for 5months or more before he finally ate the two green tiger barbs. I even starved him for 3 weeks to get him to eat them but he wouldn't. Eventually he did eat them but it took forever.
Hope that helps.