A core arrangement for a heat exchanger includes a first core layer (14). The first core layer includes first upstream (28) and downstream ends (30), first and second parting sheets (18) parallel to one another, and a plurality of adjacent fins (20, 24) disposed between the first and second parting sheets. Each of the plurality of fins extends from a surface of the first parting sheet to a surface of the second parting sheet, and longitudinally between the first upstream end and the first downstream end. The plurality of fins are further laterally arranged to define a plurality of first fluid passages (22). Each of a subset of the plurality of fins includes an internal slot (40) positioned away from the first upstream end and the first downstream end.