Pour oil to depth of 1 inch into a large deep skillet; heat to 350°. Stir together potato flakes and lemon zest. Sprinkle catfish with salt and pepper. Dip catfish in milk; dredge in potato flake mixture, pressing gently to adhere.
Fry catfish, in batches, 3 to 5 minutes on each side or until golden brown and fish just flakes with a fork. Drain on a wire rack over paper towels. Serve with Marinated Slaw.