In a bowl, combine fish and soy sauce. Marinate for about 30 minutes in the refrigerator. Drain well.
In a bowl, whisk egg until frothy.
In another bowl, combine flour, cornstarch, and salt and pepper to taste.
Dredge fish fillets in flour mixture, dip in the beaten egg, and then dredge again in flour mixture to fully coat.
In a pan over medium heat, heat about 1-inch deep of oil. Add battered fish fillets and cook, turning once or twice, until golden and cooked through. Remove from pan and drain on paper towels.
In a bowl, combine pineapple juice, vinegar, ketchup, sugar, cornstarch, and salt. Stir until smooth.
In a pan over medium heat, heat about 1 tablespoon oil. Add bell peppers and onions and cook, stirring regularly, until half cooked. Remove from pan and set aside.
In the pan, add sauce mixture and bring to a boil, whisking regularly, for about 2 to 3 minutes or until thickened.
Add bell peppers and onions and cook for about 1 minute or until tender yet crisp.
Add fried fish fillets and cook and toss quickly just until coated with sauce. Serve hot.