Castano was optioned to Triple-A Jacksonville on Friday, Jordan McPherson of the Miami Herald reports.
Castano will return to Jacksonville after he was called up June 15, posting a 4.35 ERA and 1.42 WHIP with 18 punchouts over 31 innings in six starts while in the majors. He'll likely remain one of the Marlins' top options for a potential call-up later this season. The move opened a roster spot for Cody Poteet (elbow), who was reinstated from the 15-day injured list Friday.