Trim dark parts from the fish and discard. Bring a large pot of water to a boil; drop fillets in and cook until white and flaky, 5 to 7 minutes. Drain. Transfer to a bowl and allow to cool, about 5 minutes. Flake with a fork.
Mix panko, eggs, red bell pepper, onion, celery, mustard, seafood seasoning, salt, garlic powder, and black pepper into the bowl with the flaked fish. Stir in 1/4 cup mayonnaise 1 tablespoon at a time, just until mixture holds together. Form into 6 equal-size patties.
Heat oil in a skillet over medium-high heat. Transfer patties carefully to the skillet. Fry, turning once, until golden brown, 5 to 7 minutes per side.
Whisk together 1/2 cup mayonnaise, minced garlic, lemon juice and 1/2 teaspoon seafood seasoning. Serve aioli sauce alongside patties.