Thaw fish, if frozen. In a small bowl combine olive oil, paprika, salt, cardamom, and pepper; set aside.
Cut four 18x12-inch pieces of heavy foil. fold in half to make 18x12-inch rectangles. Arrange 1 cup sweet potato slices in the center of each piece of foil.
Rinse fish; pat dry. Top sweet potatoes with a fish fillet, tucking under any thin edges of fillets. Top fish with pineapple and pepperoni. Drizzle with oil mixture. Bring together 2 opposite edges of foil; seal with a double fold. Fold remaining edges together to enclose mixture, leaving space for steam to build.
For a charcoal grill, place packets on the rack of an uncovered grill directly over medium coals. Grill for 20 to 25 minutes or until fish flakes easily when tested with a fork and potatoes are tender (do not turn packets). (For a gas grill, preheat grill. Reduce heat to medium. Place packets on grill rack. Cover and grill as above.) To serve, carefully open packets. Transfer mixture with juices to 4 shallow bowls. Sprinkle with cilantro. Makes 4 servings.