Special equipment: a deep-frying thermometer
For the tartar sauce: Mix together the lemon juice, mayonnaise, relish, capers and chives in a small bowl and season with salt to taste. Refrigerate until ready to use.
For the fried catfish: Fill a large wide pot or large deep cast-iron skillet with about 3/4 inch of vegetable oil and heat to 350 degrees F over medium-high heat. Preheat the oven to 200 degrees F and place a rack in a baking sheet.
Wash and pat the fillets dry and squeeze lemon juice over both sides. Sprinkle with seasoned salt and pepper on both sides.
Pour the milk into one wide, shallow bowl or pie plate, then add the eggs and whisk to combine. Add the fish fry breading to a second shallow bowl. Working in batches, dip the fillets in the milk and egg mixture, letting the excess drip off, then dredge the fish in the fish fry to coat thoroughly. Add the fillets to the oil and fry, turning once, until golden brown on both sides and cooked through, about 5 to 7 minutes total. Place on the rack in the baking sheet and keep warm in the oven while you dredge and fry the remaining fillets. Serve immediately with tartar sauce, hot sauce and lemon wedges.