Castano was optioned to Triple-A Jacksonville on Sunday, Jordan McPherson of the Miami Herald reports.
The 26-year-old took the loss Saturday after surrendering three runs over four innings, and he'll now make his way to Triple-A. Castano has a 5.19 ERA, 1.56 WHIP and 10:7 K:BB over 17.1 innings (four starts) for the Marlins this season.