I would wait till you upgrade. Zebrasomas would be better than a hippo, they get huge fast. My hippo I believe will be my only tang to outgrow my 215. Hippos are especially susceptible to HLLE when conditions aren't optimal. He'd be huge stress on bioload for a 55 and with only 4 ft of swimming room he'll likely be really stressed. A ctenacheous (kole tang) are also great choices for smaller tanks cuz they only get to be like 5 or 6 inches in captivity.