Sprinkle the catfish fillets with the seasoning salt, black pepper and cayenne; set aside.
Heat the oil in a large nonstick frying pan over medium-high heat until it reaches 350 degrees F on a deep-frying thermometer.
Place the cornmeal on a large plate. Dredge the fillets in the cornmeal and shake off the excess. Place the fillets in the hot oil and turn down the heat so the oil is at 300 degrees F. Fry until golden brown and cooked through, 4 minutes per side. Transfer the fish to a paper-towel-lined plate to drain.