if you want an active puffer a green spotted would be good for you, they get around 6 inches, if you could care less about activity, then a arrowhead puffer would do good, both are voracious eaters, green spotted can be kept together i believe, while arrowheads cannot. i think arrowheads get around 7 inches long. i cant think of any puffer or 8 inches that can be put in a 55.