they won't get full sized in a year but they do grow pretty fast. You'll need to upgrade from a 55 gallon if you want to keep a full grown volitans comfortably. (they can get about the size of a football with their wing span included as adults)
Lions grow faster than any other fish I know of. Volitans can reach full size 15 inches within 18 months. It will easily outgrow the 55 in a years time especially if well fed.
these fish grow fast.
Expect them to reach 8-12" in one year then reach full size in 1/5 yrs. Anywhere around 13-15", fat w/ small pec fins. Awesome fish