The Marlins optioned Castano to their alternate training site Saturday.
Castano's demotion came one day after he served as the 29th man for the Marlins' doubleheader with the Nationals. He picked up his sixth start of the season in the second game of the twin bill, giving up three runs on five hits and no walks over 4.1 innings. Since Miami has no more doubleheaders on its schedule, Castano won't be eligible to pitch for the big club in the final week of the season unless the Marlins place a player on the injured list.