Marinate the fish: In a bowl, combine the mustard, hot sauce, celery seed, garlic powder, and egg. Add the red fish to the marinade, ensuring it's fully coated. Let it marinate for a minimum of 30 minutes, or up to 4 hours in the refrigerator.
Prepare the crust: In a separate shallow dish, whisk together the cornmeal, flour, cayenne pepper, garlic powder, celery seed, black pepper, and sea salt.
Heat the oil: Pour the canola oil into a large, heavy-bottomed pot or deep fryer. Heat the oil to 375°F (190°C).
Dredge and fry the fish: Carefully dredge each piece of marinated red fish in the crust mixture, pressing to ensure a thick, even coating. Gently place the coated fish into the hot oil, ensuring not to overcrowd the pan. Fry for approximately 6 minutes per side, or until golden brown and cooked through.
Season and serve: Remove the fried fish from the oil and place on a wire rack to drain excess oil. Lightly season with a pinch of salt while still warm. Serve immediately.