The silver aro will outgrow the 55 gallon in less then 6 months. Young silvers grow about 1" per month. Once they get 9-10" they can grow 2" per month.
For the sake of the fish please be prepared for this.
Here is a picture of a silver aro I adopted. He was purchased at 6" and kept in a 90 gallon 48" long tank for a year and a half. He is now 22" and his deformed back is most likely from being kept in a tank to small.