Neonmadness, a black aro that size ought to do fine in a tank those dimensions. Be sure to keep all the gap spaces on the top of the tank covered and your water qualty good and consistant. Black arows at that size are quite more sensative and jumpy... compared to it's close cousin, the silver aro. Blacks also grow at a much more slower rate than the silvers do. Be sure to keep us posted on their progress and good luck!