Combine first 6 ingredients in a small bowl; stir well.
Rinse fillets, and pat dry with paper towels to remove excess moisture. Sprinkle paprika mixture evenly on both sides of fillets; rub seasonings in gently.
Place a 12-inch cast iron skillet over high heat until hot. Add 1/2 tablespoon butter, and tilt pan to coat bottom. Add 2 fillets, and cook 2 minutes. Reduce heat to medium; turn, and cook 4 minutes or until fish flakes easily when tested with a fork. Remove from pan, and keep warm. Repeat procedure with remaining butter and fillets. Serve with lemon slices, if desired.