MICROWAVE butter in large microwaveable bowl on HIGH 45 sec. or until melted. Add marshmallows; toss to coat. Microwave 1 min. 30 sec. or until marshmallows puff, stirring every 45 sec. until marshmallows are completely melted and mixture is well blended. Stir in cereal.
UNROLL fruit snack sheets. Press 3 Tbsp. cereal mixture onto each sheet to within 1/2 inch of edge. Place 4 fish down center of each sheet; roll up tightly.
CUT each roll-up into 4 pieces.